Implement autorest x-ms extensions

[lmazuel]

x-ms-extensions are defined here:
https://github.com/Azure/autorest/blob/master/docs/extensions/readme.md

We need to re-implement them in Autorest v3 generators.

Assuming remodeler passes the metadata, this is the extensions I believe are language specific:

• x-ms-skip-url-encoding
• x-ms-enum
• x-ms-client-flatten
• x-ms-parameterized-host
• x-ms-error-response
• x-ms-odata (Python doesn't plan to support)
• x-ms-azure-resource (ARM specific)

I expect the remodeler to do something about it:

• x-ms-parameter-grouping (should create the group in the YAML)
• x-ms-parameter-location (should be in the right place in the YAML)
• x-ms-paths (should be a regular path on consumption)
• x-ms-client-name (new field preferred name: name from Swagger or x-ms-client-name)
• x-ms-discriminator-value (new field preferred value: value from Swagger or x-ms-disc-value)

Needs to be discussed if we drop support:

• x-ms-external
• x-ms-mutability
• x-ms-request-id
• x-ms-client-request-id

Dedicated issues:

• x-ms-pageable: Implement x-ms-pageable #7402
• x-ms-long-running-operation: Implement x-ms-long-running-operation #7405

[lmazuel]

Done