Provide separate starters for client and server

[DanielLiu1123]

As mentioned in this comment, some services only depend on the gRPC client or server. Currently, spring-grpc only provides one starter. For these use cases, users need to manually add grpc-stub or grpc-services dependencies.

I think these are very two different use cases and it’s worth having spring-grpc provide two additional starters:

spring-grpc-client-spring-boot-starter # only client
spring-grpc-server-spring-boot-starter # only server

The existing starter should still be available, it includes both client and server starters:

spring-grpc-spring-boot-starter
├── spring-grpc-client-spring-boot-starter
└── spring-grpc-server-spring-boot-starter

[dsyer]

I'm not convinced that the grpc-services should be included yet. Personally I always use it, but wouldn't it be better to leave it as optional? We might end up with just 2 starters that way as well - there's no need for the combo because client is a superset of server.