Engine request improvement

[sekimondre]

Improve semantics for requesting wc methods from the engine and remove completion blocks for response handling. Provides an abstraction to prevent creating request objects where the method doesn't match the parameters sent.

• Adds WCMethod to pass the method parameters to the relayer and simplify the request call.
• Removes the need to always match parameters pairs when creating a WCRequest from the engine.
• Changes the completion parameter on the WalletConnectRelaying request method to become optional. This change prepares the code to gradually remove all completion handlers used for responses in the engines.