[Access] Add REST endpoint to get an accounts flow balance

[AndriiDiachuk]

In this PR a new REST endpoint was added that returns the account's flow balance using the new GetAccountBalance method in the backend.

Closes: #6001
Related PR: onflow/flow#1491

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 51.66667% with 29 lines in your changes missing coverage. Please review.

Project coverage is 41.55%. Comparing base (f296f51) to head (201414e).

Files	Patch %	Lines
engine/access/rest/apiproxy/rest_proxy_handler.go	0.00%	13 Missing ⚠️
engine/access/rest/request/get_account_balance.go	68.42%	6 Missing ⚠️
engine/access/rest/routes/account_balance.go	68.42%	4 Missing and 2 partials ⚠️
engine/access/rest/request/request.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6253      +/-   ##
==========================================
- Coverage   41.55%   41.55%   -0.01%     
==========================================
  Files        1993     1995       +2     
  Lines      141665   141724      +59     
==========================================
+ Hits        58868    58888      +20     
- Misses      76667    76705      +38     
- Partials     6130     6131       +1     

Flag	Coverage Δ
unittests	41.55% <51.66%> (-0.01%)	⬇️

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[Guitarheroua]

Looks good, has couple comments to fix.

[peterargue]

looking good. added a couple small comments

[peterargue]

after adding the test, I think this is ready to go