Allow generalising over RPC implementation (#634)

* WIP generalising RPC client

* WIP: non-object-safe RpcClientT.. aah generics everywhere

* WIP object-safe RpcClientT trait and no more extra generics

* Get core things compiling again with object-safe RpcClientT trait

* Make jsonrpsee optional and get test-runtime working again

* cargo fmt

* add RpcParams object to enforce correct formatting of rps params

* Wee tweaks

* clippy fixes

* cargo fmt

* TWeak a few types

* make sure we get jsonrpsee-types, too

* Add examples for rpc_params/RpcParams

* more doc tweaks

* remove a now unneeded dev note

* Option<Box<RawValue>> instead to avoid allocations in some cases

* update docs

* tweak RpcClientT trait docs

* Tweak docs around RpcClient and RpcClientT. Don't expose RpcClientT directly

* more doc tweaking about RpcParams and undo decision not to expose RpcParamsT

* Doc tweak

Co-authored-by: Alexandru Vasile <60601340+lexnv@users.noreply.github.com>

* more doc tweaks

* Fix doc thing

* Add an example of injecting a custom RPC client

* Fix a typo

* Address clippy things in example

* Fix a silly typo

* another clippy fix

* rpc_params to panic instead of returning a result, like serde_json::json, and deref on Rpc<T>

* fix docs

Co-authored-by: Alexandru Vasile <60601340+lexnv@users.noreply.github.com>

testing/test-runtime/Cargo.toml

@@ -13,3 +13,4 @@ subxt = { path = "../../subxt" }
 sp-core = "6.0.0"
 tokio = { version = "1.8", features = ["macros", "rt-multi-thread"] }
 which = "4.2.2"
+jsonrpsee = { version = "0.15.1", features = ["async-client", "client-ws-transport"] }