* feat: enable self signed jwt for gapic clients

* resolve comments

* update gax version

* update goldens

* update golden files

Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>

This includes:
1) Proper GRPC transcoding
2) Mimimal REST Operations (go make it compilable)
3) Rely on resource names for tests values generation (to not fail http url path validation)
4) Empty Server Streaming method implementation

1) Fully backward compatible with grpc-only GAPIC
2) Strictly speaking not fully backward compatible with rest-only REGAPIC (but is compatible for practical cases). See below for details.
3) Introduces the concept of default transport (is necessary to preserve backward-compabitility). The default behavior assumes grpc transport.

Needed to unblock DIREGAPIC LRO. To be reviewed after submission

…#835)