rpc: library middleware support

[meowsbits]

This issue proposes adding support to the rpc package for library-level middleware, akin to

• https://github.com/gin-gonic/gin#using-middleware
• https://github.com/gorilla/mux#middleware

This would enable library-use level implementations of header customization, authentication and authorization schemes, and even custom request/response content processing (eg. return receipts with transactions).