[feat] Intro GeneratorMetadata (stability index)

[jimschubert]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

GeneratorMetadata offers an immutable object created via Builder pattern
which allows generators to explicitly define their stability (stable,
beta, experimental, deprecated) as well as a message to be shown during
generation.

This is a step toward:

• Fleshing out the "Core" artifact (Create a "core" artifact #845)
• Providing a place to encapsulate feature-oriented metadata (Create reusable feature metadata for all generators #840)
• Providing a means to communicate end of life scheduling (Discuss: end of life scheduling for outdated templating #116)

This new structure, specifically the Stability property, allows us to
offer future enhancements such as allowing users to filter down to only
"Stable" generators via CLI, and eventually any compat table (see #503).

See project 5 for other potentially related work.

---

This is an addition to what was introduced for the C# 2.0 generator in #2804. I had opened #2786, not realizing that the generator hadn't been deprecated as discussed in #116. After consideration, I decided to have something typed which would explicitly define the generator as deprecated in code.

Example:

$ ./bin/csharp-dotnet2-petstore.sh
# START SCRIPT: ./bin/csharp-dotnet2-petstore.sh
[main] INFO  o.o.codegen.DefaultGenerator - OpenAPI Generator: csharp-dotnet2 (client)
[main] WARN  o.o.codegen.DefaultGenerator - Generator 'csharp-dotnet2' is considered deprecated.
[main] WARN  o.o.codegen.utils.ModelUtils - Multiple schemas found in content, returning only the first one
[main] WARN  o.o.codegen.utils.ModelUtils - Multiple schemas found in content, returning only the first one
[main] WARN  o.o.codegen.utils.ModelUtils - Multiple schemas found in content, returning only the first one
[main] WARN  o.o.codegen.utils.ModelUtils - Multiple schemas found in content, returning only the first one
[main] WARN  o.o.codegen.utils.ModelUtils - Multiple schemas found in content, returning only the first one
[main] WARN  o.o.codegen.utils.ModelUtils - Multiple schemas found in content, returning only the first one
[main] WARN  o.o.codegen.utils.ModelUtils - Multiple schemas found in content, returning only the first one
[main] WARN  o.o.codegen.utils.ModelUtils - Multiple schemas found in content, returning only the first one
[main] WARN  o.o.codegen.utils.ModelUtils - Multiple schemas found in content, returning only the first one
[main] WARN  o.o.codegen.utils.ModelUtils - Multiple schemas found in content, returning only the first one
[main] WARN  o.o.codegen.utils.ModelUtils - Multiple schemas found in content, returning only the first one
[main] WARN  o.o.codegen.utils.ModelUtils - Multiple schemas found in content, returning only the first one
[main] WARN  o.o.codegen.utils.ModelUtils - Multiple schemas found in content, returning only the first one
[main] WARN  o.o.codegen.utils.ModelUtils - Multiple schemas found in content, returning only the first one
[main] WARN  o.o.codegen.utils.ModelUtils - Multiple schemas found in content, returning only the first one
[main] WARN  o.o.codegen.utils.ModelUtils - Multiple schemas found in content, returning only the first one
[main] WARN  o.o.c.l.CSharpDotNet2ClientCodegen - Per Microsoft Product Lifecycle (https://support.microsoft.com/en-us/lifecycle/search?sort=PN&alpha=.NET%20Framework&Filter=FilterNO), support for .NET Framework 2.0 ended in 2011 so there may be security issues using the auto-generated C# 2.0 source code.

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[jimschubert]

Question:

This creates some redundancy in the "list" CLI command, where some generators previously encoded the stability into the name of the generator:

    - scala-httpclient-deprecated (deprecated)
    - swift2-deprecated (deprecated)
    - swift3-deprecated (deprecated)

I think this is fine for now, since machine parsing should be done via the list --short option. It seems to me that we might want an UNMAINTAINED option to differentiate between something which is deprecated (i.e. "this will be removed at some point") versus something which is either low priority or being kept with no intention to remove. If that's not the case, we may want to consider explicitly defining end of life for the above mentioned deprecated generators.

[jimschubert]

cc @OpenAPITools/generator-core-team
for visibility @OpenAPITools/openapi-generator-collaborators

[jimschubert]

NOTE: We could apply Stability to other things as well, such as the templating engine adapters.