feat(rpc): make fetch customizable

[homura]

Description

Fixes #573

This PR makes the fetch in the @ckb-lumos/rpc become customizable

Type of change

Please delete options that are not relevant.

• [x New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• unit test

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[codecov]

Codecov Report

Merging #574 (48fbe3c) into develop (68e4f5b) will increase coverage by 0.80%.
The diff coverage is 76.92%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #574      +/-   ##
===========================================
+ Coverage    85.88%   86.69%   +0.80%     
===========================================
  Files          116      116              
  Lines        23429    23434       +5     
  Branches      2297     2343      +46     
===========================================
+ Hits         20122    20315     +193     
+ Misses        3267     3077     -190     
- Partials        40       42       +2     

Files	Coverage Δ
packages/rpc/src/method.ts	85.71% <100.00%> (+0.32%)	⬆️
packages/rpc/src/index.ts	75.41% <53.84%> (+0.98%)	⬆️

... and 4 files with indirect coverage changes

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