Add CopyToAsync optimized implementations for StreamPipeReader

[manandre]

Closes #51972

/cc @davidfowl

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, to please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[davidfowl]

Thanks for the contribution @manandre. I'd prefer it if we didn't expose any code with the base implementation and instead optimized for calling as little functions inside of the CopyToAsync implementations inside of the StreamPipeReader itself.

I expected the calls to CopyToAsync to basically enumerate from readHead to readTail and copy that to the underlying destination (PipeWriter or Stream), and then in the Stream case, delegate completely to the Stream implementation. For the PipeWriter case, what you're doing is fine.

[davidfowl]

This change is looking great! We need to figure out the tests.

[davidfowl]

I'm very happy with this change @manandre!

[davidfowl]

Thanks @manandre ! I will be looking for more Pipelines work to give you 😉