Add GetAccountBalance endpoint to grpc server

[illia-malachyn]

Closes: #741

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 24.52830% with 120 lines in your changes missing coverage. Please review.

Project coverage is 51.03%. Comparing base (d91be5b) to head (6e6b594).
Report is 167 commits behind head on master.

Files with missing lines	Patch %	Lines
access/grpc/mocks/RPCClient.go	17.14%	106 Missing and 10 partials ⚠️
access/grpc/client.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #742       +/-   ##
===========================================
- Coverage   61.97%   51.03%   -10.94%     
===========================================
  Files          27       35        +8     
  Lines        3734     5410     +1676     
===========================================
+ Hits         2314     2761      +447     
- Misses       1231     2309     +1078     
- Partials      189      340      +151     

Flag	Coverage Δ
unittests	51.03% <24.52%> (-10.94%)	⬇️

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[illia-malachyn]

@janezpodhostnik please, merge it in