Add DPG testserver tests for developer driven evolution

[pshao25]

Description

Add DGP testserver tests for developer driven evolution

Addresses #1944

Test Cases

Service Methods

• getModel - protocol method: public virtual Response GetModel(string mode, RequestContext context = null)
• getModel - grow-up method: public virtual Response<Product> GetModelValue(string mode, CancellationToken cancellationToken = default)
• postModel - protocol method: public virtual Response PostModel(string mode, RequestContent content, RequestContext context = null)
• postModel - grow-up method: public virtual Response<Product> PostModel(string mode, Input input, CancellationToken cancellationToken = default)
• getPages - protocol method: public virtual Pageable<BinaryData> GetPages(string mode, RequestContext context = null)
• getPages - grow-up method: public virtual Pageable<Product> GetPagesValues(string mode, CancellationToken cancellationToken = default)

LRO grow-ups moved to #2027.

Relevant docs

https://github.com/Azure/autorest.testserver/wiki/Data-plane-codegen-testing#developer-driven-evolution
https://github.com/Azure/autorest.testserver/blob/main/swagger/dpg-customization.json

Checklist

To ensure a quick review and merge, please ensure:

• The PR has a understandable title and description explaining the why and what.
• The PR is opened in draft if not ready for review yet.
  • If opened in draft, please allocate sufficient time (24 hours) after moving out of draft for review
• The branch is recent enough to not have merge conflicts upon creation.

Ready to Land?

• Build is completely green
  • Submissions with test failures require tracking issue and approval of a CODEOWNER
• At least one +1 review by a CODEOWNER
• All -1 reviews are confirmed resolved by the reviewer
  • Override/Marking reviews stale must be discussed with CODEOWNERS first

[annelo-msft]

@pshao25, I wasn't able to find the swagger file the client was generated from. Could you provide a link to this in the PR description?

[ShivangiReja]

@annelo-msft - Here is the swagger link https://github.com/Azure/autorest.testserver/blob/main/swagger/dpg-customization.json. This is a test server project. All languages use test server swagger to generate code and add test cases to verify that the generated result is correct or not.

[AlexanderSher]

LGTM.

[annelo-msft]

Let's wait to check this in until we have resolution on what the grow-up methods should look like.

[annelo-msft]

Added a few comments and checked off completed APIs in the description checklist. This is getting close!

[annelo-msft]

Only things left are updating to GetPagesValues and using the model's on serializer for creating the RequestContext.

[annelo-msft]

Thanks for your perseverance on this task!