feat: add grpc middleware.

This is a middleware implementation that lets you implement your middleware
outside of Dex itself, in a separate gRPC server.

Signed-off-by: Alastair Houghton <alastair@alastairs-place.net>

Makefile

@@ -83,6 +83,7 @@ proto: bin/protoc bin/protoc-gen-go
 	@cp api/v2/*.proto api/
 	@./bin/protoc --go_out=plugins=grpc:. --plugin=protoc-gen-go=./bin/protoc-gen-go api/*.proto
 	@./bin/protoc --go_out=. --plugin=protoc-gen-go=./bin/protoc-gen-go server/internal/*.proto
+	@./bin/protoc --go_out=plugins=grpc:. --plugin=protoc-gen-go=./bin/protoc-gen-go middleware/grpc/api/*.proto
 
 .PHONY: verify-proto
 verify-proto: proto