`EventSignature` model & Contract middleware to cache event signatures by signature

[github-actions[bot]]

Separate the event signatures from the 4byte function signatures and add middleware on the Contract model to add any relevant signatures to model. Then update the ContractEvent to decode getPastLogs and subscribeLogs calls when possible.

To summarize

• EventSignature model
• Middleware to auto add event signature from contract abi
• Middleware to auto decode ContractEvent if no returnValues using the EventSignature Contract.eventAbiBySignature info closes #111