Support v1alpha gRPC reflection

[ttkzhed]

Hi, we just tried out tonic 0.12.0 and we noticed it's using v1 reflection instead of v1alpha (related tonic change: #1701).

That breaks usage of common gRPC tools like Postman and Kreya.

There doesn't seem a way to support v1alpha reflection anymore, are we missing anything?
Otherwise, would it make sense to support this use case via opt-in?

E.g.

tonic_reflection::server::Builder::configure()
  .enable_v1alpha_reflection()
  ...
  .build()

[ttkzhed]

There's a Pull Request with a potential fix: #1787

[ttkzhed]

Fixed in #1888!

[4F2E4A2E]

@v0.12.3 using build_v1alpha() did it for me