feat: client trait + porting client proc macros (#199)

* draft: client trait sketch...

* use async_trait instead of `Box<Future>`

* refactor client trait

* [types]: shared client types.

* integrate with `jsonrpsee_proc_macros::rpc_api`

* Update proc-macros/src/lib.rs

* integrate with proc macros

* fix: hacky support for generic types in proc macro

* fix: make the examples work.

* trait: error associated type

* Update types/src/traits.rs

* client trait: make object safe.

Make the trait usable as a trait object i.e, `Box<dyn Trait>`

* client trait: remove `Self::Sized`

* add tests for proc macros.

* fix build

* fmt

* [client trait]: remove unused associated error typ

* [proc macros]: separate enum variant each return_t

* add tracking issue to `TODO`

* doc(client trait): improve documentation.

* separate trait for subscribing client

* add documentation

* proc macros: remove debug impl

* fix compile warns

* address grumbles: remove fn process_response

* Update types/src/client.rs

Co-authored-by: David <[email protected]>

* Update types/src/client.rs

Co-authored-by: David <[email protected]>

* Update types/src/client.rs

Co-authored-by: David <[email protected]>

* Update types/src/client.rs

Co-authored-by: David <[email protected]>

* Update types/src/client.rs

Co-authored-by: David <[email protected]>

* remove old proc macro tests

* address grumbles: shorter lines

Co-authored-by: David <[email protected]>