client: set grpc-accept-encoding to full list of registered compressors

[jronak]

Part of #2786 (rest fixes in follow-up PRs)

Description:
In the current implementation, we forward grpc-accept-encoding only when the client sets compression on the outgoing RPCs. With this pull request, all the compressors registered with encoding.RegisterCompressor are added under the grpc-accept-encoding header for the outgoing RPCs.

When the client enables compression using deprecated grpc.WithCompressor, stick to the old behaviour of setting the grpc-accept-header header to the outgoing compressor.

RELEASE NOTES:

• client: set grpc-accept-encoding header with all registered compressors

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: jronak / name: Ronak Jain (8018099)

[dfawley]

Thanks for the PR! Overall it looks pretty good. Comments inline, and one higher level comment:

I'm at least a little nervous about enabling this by default. It may be better to have this controlled by a dial option that either lists the codecs the user wants to expose or enables/disables this functionality. I'll get back to you about this part.

[dfawley]

Sorry for the long delay here. I discussed this with the other language leads and we think a change like this should be good. If it isn't too much trouble, would you mind putting this behavior behind an environment variable (turned on by default), so that, in case users run into trouble in the next couple releases, they can disable it? See: https://github.com/grpc/grpc-go/blob/master/internal/envconfig/envconfig.go

Thank you!

[dfawley]

LGTM. Could you please take a pass on this, too, @easwars ?