Refactor rpc handlers and tests

[pnowosie]

Fixes #1750

All handlers and related helper methods was moved from rpc/handlers to the corresponding "category" file, e.g. block, transaction, trace...

Handlers tests were refactor the same way.

I introduced new files in rpc/ folder:

• chain.go - with ChainId handler
• class.go - Class* handlers
• contract.go - Nonce & StorageAt
• estimate_fee.go
• rpc/helpers.go which contains helper functionality used in more than one "category".

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 59.87762% with 459 lines in your changes are missing coverage. Please review.

Project coverage is 74.53%. Comparing base (e43b0f3) to head (19169ee).

Files	Patch %	Lines
rpc/transaction.go	66.77%	83 Missing and 17 partials :warning:
rpc/trace.go	44.17%	78 Missing and 13 partials :warning:
rpc/estimate_fee.go	0.00%	74 Missing :warning:
rpc/events.go	44.44%	60 Missing and 5 partials :warning:
rpc/simulation.go	24.41%	61 Missing and 4 partials :warning:
rpc/block.go	73.84%	29 Missing and 5 partials :warning:
rpc/helpers.go	78.70%	17 Missing and 6 partials :warning:
rpc/class.go	92.59%	3 Missing and 1 partial :warning:
rpc/sync.go	91.66%	1 Missing and 1 partial :warning:
rpc/state_update.go	98.50%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1768      +/-   ##
==========================================
- Coverage   74.81%   74.53%   -0.29%     
==========================================
  Files          90       96       +6     
  Lines        8177     8177              
==========================================
- Hits         6118     6095      -23     
- Misses       1543     1565      +22     
- Partials      516      517       +1     

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