Creating a common base for chain clients (`BitcoinClient` and `EVMChainClient` currently)

[lumtis]

Describe the Issue

EVMChainClient and BitcoinClient has a lot in common aside from the logic to actually observe transactions

We should consider grouping the common logic under a chain base, this would also be reused for future chains

This eventually can be defined in the chain_client package from #2020 by create a ChainClientBase

Some of the tasks:

• Group common logic in in NewBitcoinClient and NewEVMChainClient
• A function initializing logger, core_context, etc..

Bonus: renaming client into observer, since this is the component observing external chain transaction