New plugin: useExtendedValidation and implementation of @oneOf directive

[dotansimha]

Background for this plugin:

At the moment, the existing phases of the requests pipeline are consist of: parse, validate, contextBuilding, execute.

Variables parsing happens on a separate flow (as part of the request pipeline), and it's being provided to execute externally. Input coercion happens during execution, as an internal step. The implication of this is the fact that validation rules only have access to the AST of the documents (and to the validation context in general), but not to variables or execution params.

Modern implementation often needs to access variables in order to provide a more complete validation phase.

To work around that, we can implement in envelop a plugin for extended validation. We follow the same ValidationRule interface, with the addition of ExecutionArgs provided. And Envelop makes sure to pass run that before execute runs.
The flow is the same as validate and it uses the same visitors flow, but allows developers to write more simplified code, and do more enriched validations.

As a simple example for this POC, I implemented the basics of this plugin and also implemented @oneOf directive for adding support for input unions.

More ideas that we might be able to implement:

• Allow having arguments/directives for scalars at the level of the usage, something like field: String @max(5).
• Allow having validation rules that are aware of the user: field: String! @authorization(role: "test").

This might also introduce another way of doing directive-based validations, since it does not require composing resolvers.

[theguild-bot]

The latest changes of this PR are available as alpha in npm (based on the declared changesets):

@envelop/core@0.1.4-alpha-831a172.0
@envelop/extended-validation@0.0.1-alpha-831a172.0
@envelop/types@0.1.4-alpha-831a172.0

[codecov-commenter]

Codecov Report

Merging #149 (d1a4665) into main (e25d704) will increase coverage by 2.48%.
The diff coverage is 98.43%.

@@            Coverage Diff             @@
##             main     #149      +/-   ##
==========================================
+ Coverage   75.49%   77.97%   +2.48%     
==========================================
  Files          20       24       +4     
  Lines         510      572      +62     
  Branches       89      105      +16     
==========================================
+ Hits          385      446      +61     
- Misses        125      126       +1     

Impacted Files	Coverage Δ
packages/plugins/extended-validation/src/common.ts	92.30% <92.30%> (ø)
packages/core/src/create.ts	75.00% <100.00%> (+0.78%)	⬆️
packages/plugins/extended-validation/src/index.ts	100.00% <100.00%> (ø)
packages/plugins/extended-validation/src/plugin.ts	100.00% <100.00%> (ø)
...es/plugins/extended-validation/src/rules/one-of.ts	100.00% <100.00%> (ø)
packages/testing/src/index.ts	91.17% <100.00%> (ø)

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