gRPC plugin API for signers

[matt-allan]

I read through #1907 and #1020 which discuss adding a gRPC API for connectors. I think that would be great. I'm wondering if a gRPC plugin API would make sense for the Signer interface too.

My only option right now for signing keys is to either store the key material in the database or use Vault. I would prefer to use my cloud's KMS or secret manager. A gRPC API would allow adding the signer without pulling cloud vendor SDKs into Dex. The main downside is the user has to run a sidecar service.

Config would be something like this:

signer:
  type: grpc
  config:
    addr: 127.0.0.1:5557
    tlsCert: examples/grpc-client/server.crt
    tlsKey: examples/grpc-client/server.key
    tlsClientCA: examples/grpc-client/ca.crt

[nabokihms]

Hi @matt-allan, we are investigating the right way to plug connectors / storages to Dex. The latest idea was described in #4578