refactor(core): centralize blockchain_rpc methods into a single enum

[0xdeafbeef]

Reduce boilerplate for metrics.
Split rpc on several files.

[codecov]

Codecov Report

Attention: Patch coverage is 69.73684% with 23 lines in your changes missing coverage. Please review.

Project coverage is 45.22%. Comparing base (313a8ed) to head (d9253aa).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
core/src/blockchain_rpc/service/mod.rs	67.56%	12 Missing ⚠️
util/src/metrics/histogram_guard.rs	70.00%	8 Missing and 1 partial ⚠️
core/src/blockchain_rpc/service/util.rs	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #659      +/-   ##
==========================================
+ Coverage   45.14%   45.22%   +0.08%     
==========================================
  Files         291      292       +1     
  Lines       55030    55046      +16     
  Branches    55030    55046      +16     
==========================================
+ Hits        24842    24897      +55     
+ Misses      28946    28913      -33     
+ Partials     1242     1236       -6     

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