Support grpc.reflection.v1alpha.ServerReflection

[sashayakovtseva]

Hi.

I see you've explicitly removed v1alpha support in #58.
However, many third party tools (e.g. postman, k6) are still using v1alpha reflection. Essentially now anyone who depend on that tools cannot upgrade to SpiceDB v1.26.0.

I suggest to follow https://github.com/grpc/grpc-go approach when you support both server versions for some reasonable amount of time.

[vroldanbet]

Thanks for reporting! I've opened #58 and authzed/spicedb#1718