Improve client middleware documentation

[koevskinikola]

Description

There is very little information on how Camunda users can set up custom client middleware. The only mention is here.

One reason for this lack of documentation is that client middleware is implementation dependant, i.e. depending on the (Zeebe) client implementation (ex. NodeJS, Java, Spring, Go, CLI, etc.), the middleware API will vary depending on the (REST) HTTP client that is used, as well as the gRPC client implementation.

Client middleware is currently supported for:

• Zeebe java client / Spring SDK
  • For REST through the Apache HttpClient 5 AsyncExecChainHandler interface.
  • For gRPC through the grpc-java ClientInterceptor interface.

*AC(

• [ ] Add example implementations of common use-cases for custom middleware here (ex. custom request headers).
• [ ] TBD