[Access] OK response should not contain error message

[illia-malachyn]

Branched off #6907

• Pass object by pointer rather than by value, so that it is not encoded with zeros but omitted in JSON
• Move data providers' models to data providers package.
• Replace Build() function with factory method pattern New...()

Closes #7009 #7039

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 64.58333% with 170 lines in your changes missing coverage. Please review.

Project coverage is 41.06%. Comparing base (121c800) to head (0f3da98).
Report is 14 commits behind head on master.

Files with missing lines	Patch %	Lines
...s/rpc/backend/transaction_subscription_metadata.go	64.03%	31 Missing and 10 partials ⚠️
engine/access/rest/common/utils.go	47.91%	24 Missing and 1 partial ⚠️
engine/access/rest/websockets/controller.go	55.55%	19 Missing and 1 partial ⚠️
cmd/access/node_builder/access_node_builder.go	0.00%	17 Missing ⚠️
cmd/observer/node_builder/observer_builder.go	0.00%	17 Missing ⚠️
.../access/rpc/backend/backend_stream_transactions.go	72.13%	12 Missing and 5 partials ⚠️
engine/access/rest/util/converter.go	0.00%	8 Missing ⚠️
.../rest/websockets/data_providers/events_provider.go	61.53%	4 Missing and 1 partial ⚠️
access/api.go	0.00%	4 Missing ⚠️
...ockets/data_providers/account_statuses_provider.go	72.72%	2 Missing and 1 partial ⚠️
... and 8 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7037      +/-   ##
==========================================
- Coverage   41.14%   41.06%   -0.09%     
==========================================
  Files        2132     2132              
  Lines      187048   187033      -15     
==========================================
- Hits        76966    76808     -158     
- Misses     103633   103774     +141     
- Partials     6449     6451       +2     

Flag	Coverage Δ
unittests	41.06% <64.58%> (-0.09%)	⬇️

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[illia-malachyn]

Some tests might fail because of #6907

[Guitarheroua]

This one looks good to me!