Add GRPC client

[withinboredom]

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Describe the feature

Dapr supports GPRC which the PHP SDK could benefit from.

Release Note

RELEASE NOTE: ADD GPRC client

[withinboredom]

Currently waiting for 1.36 to be released of grpc/grpc, though I do have a functioning local branch that uses 1.35. gRPC is going to be interesting and increases the test surface area considerably.

It appears you'd have to have the grpc/protobuf extensions installed even if you want to only use the REST API of Dapr which is disappointing. I'm hoping to figure out some way around that, likely by providing a separate package.

[litan1106]

Looking for this GRPC feature

[withinboredom]

It's coming @litan1106, this is my highest priority item and I'd like to see it ready for production by the end of May.