Add mempool enpoints

[YeahNotSewerSide]

Description

Add mempool endpoints to the REST API

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How Has This Been Tested?

Added unit tests for the new endpoints

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• New and existing E2E tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have not added any vulnerable dependencies to my code

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 73.00000% with 27 lines in your changes missing coverage. Please review.

Project coverage is 60.48%. Comparing base (77e0426) to head (c7e9c5e).

Files with missing lines	Patch %	Lines
api/node/node.go	85.71%	10 Missing and 2 partials ⚠️
api/utils/http.go	0.00%	9 Missing ⚠️
cmd/thor/main.go	0.00%	2 Missing ⚠️
txpool/tx_pool.go	33.33%	2 Missing ⚠️
api/api.go	0.00%	1 Missing ⚠️
cmd/thor/utils.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1035      +/-   ##
==========================================
+ Coverage   60.44%   60.48%   +0.04%     
==========================================
  Files         219      219              
  Lines       22778    22876      +98     
==========================================
+ Hits        13768    13837      +69     
- Misses       7914     7940      +26     
- Partials     1096     1099       +3     

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[freemanzMrojo]

In general LGTM, just left some comments about some suggested changes

Requirements changed

[otherview]

Minor nits - would be nice to fix the receiver mismatch, other comments are smaller things.