[Access] Created new gRPC methods for GetAccountBalance and GetAccountKeys.

[AndriiDiachuk]

In this PR new gRPC methods GetAccountBalance and GetAccountKeys were added.

Closing issue: #5999
Related PRs: onflow/flow#1475 onflow/flow-emulator#707

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 8.23666% with 791 lines in your changes missing coverage. Please review.

Project coverage is 41.39%. Comparing base (b63957c) to head (2a695a6).

Files	Patch %	Lines
engine/access/mock/access_api_client.go	0.00%	144 Missing ⚠️
engine/access/mock/access_api_server.go	0.00%	108 Missing ⚠️
access/mock/api.go	0.00%	106 Missing ⚠️
access/handler.go	0.00%	102 Missing ⚠️
engine/access/apiproxy/access_api_proxy.go	0.00%	84 Missing ⚠️
engine/access/rpc/backend/backend_accounts.go	47.18%	51 Missing and 24 partials ⚠️
engine/execution/computation/query/executor.go	0.00%	37 Missing ⚠️
...ngine/execution/computation/query/mock/executor.go	0.00%	35 Missing ⚠️
module/execution/mock/script_executor.go	0.00%	35 Missing ⚠️
fvm/environment/mock/account_info.go	0.00%	18 Missing ⚠️
... and 4 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6144      +/-   ##
==========================================
- Coverage   41.58%   41.39%   -0.19%     
==========================================
  Files        1976     1976              
  Lines      139757   140591     +834     
==========================================
+ Hits        58112    58201      +89     
- Misses      75609    76335     +726     
- Partials     6036     6055      +19     

Flag	Coverage Δ
unittests	41.39% <8.23%> (-0.19%)	⬇️

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[peterargue]

overall this is looking good. added a couple comments related to my comments in the proto changes

[peterargue]

replied to your question about getting the available balance when using execution-only-mode. sorry for the churn on that request.

[peterargue]

added a few more comments. after this I think it's ready to go.

[peterargue]

Looks good. @Guitarheroua can you take a look as well. after we get the merged emulator commit added, I'll approve and merge.

[Guitarheroua]

Looks good! Just a minor comment about documentation.

[peterargue]

@AndriiDiachuk I merged the emulator PR, please update this PR with the commit and I'll approve and merge

[AndriiDiachuk]

@AndriiDiachuk I merged the emulator PR, please update this PR with the commit and I'll approve and merge

Made the changes, but please check if I made any mistakes.

[janezpodhostnik]

Looks good to me.

Great work.

I'll change some of the types in #6201