refactor: create base signer and signer structs to group common data of EVM/Bitcoin signer and observer

[ws4charlie]

Description

This refactor created three base structs below. The final goal is to use these reusable base structs to simply existing EVM/Bitcoin signers and observers.

• Created base Signer as the base struct for all chain signers.
• Created base Observer as the base struct for all chain observers.
• Created base Logger struct that contain commonly used loggers by observers.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Include instructions and any relevant details so others can reproduce.

• Tested CCTX in localnet
• Tested in development environment
• Go unit tests
• Go integration tests
• Tested via GitHub Actions

Checklist:

• I have added unit tests that prove my fix feature works

[codecov]

Codecov Report

Attention: Patch coverage is 95.85253% with 9 lines in your changes missing coverage. Please review.

Project coverage is 68.37%. Comparing base (d4a9f7d) to head (2e68884).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2344      +/-   ##
===========================================
+ Coverage    68.06%   68.37%   +0.30%     
===========================================
  Files          299      302       +3     
  Lines        18971    19178     +207     
===========================================
+ Hits         12913    13112     +199     
- Misses        5421     5426       +5     
- Partials       637      640       +3     

Files	Coverage Δ
zetaclient/chains/base/signer.go	100.00% <100.00%> (ø)
zetaclient/chains/bitcoin/observer/observer.go	14.31% <100.00%> (ø)
zetaclient/chains/bitcoin/signer/signer.go	19.35% <100.00%> (ø)
zetaclient/chains/evm/signer/signer.go	53.97% <100.00%> (ø)
zetaclient/chains/evm/observer/observer.go	32.04% <66.66%> (ø)
zetaclient/chains/base/logger.go	95.12% <95.12%> (ø)
zetaclient/chains/base/observer.go	95.52% <95.52%> (ø)

[lumtis]

LGTM, just a few comment regarding documentation