Add Http headers for RPC and PubSub Clients

[ozanselte]

Problem

Some RPC and WSS servers need specific HTTP headers. It is possible to solve the problem for RPC client but I could not find a way for WebSocket besides doing my own connections and requests. So here is the solution for those problems.

Summary of Changes

Added reqwest::header parameters for some methods.
Created new methods for rpc-client and added new parameters for pubsub-client.

Fixes #3917

[t-nelson]

this cannot be accepted as is. it breaks public api. i'd recommend a strategy similar to the RpcClient::new_sender interface, but it's going to be major surgery

[ozanselte]

Yes this PR breaks PubSubClient but I did not get it how does it break RpcClient library?
Since this PR is not merged, I am using new_sender on my projects.
@t-nelson