Add group-id parameter to GET /v2/transactions

[agodnic]

Summary

Issue: #135

This pull request adds the group-id parameter to the endpoint GET /v2/transactions (this feature has already been implemented for Nodely in AlgoNode#2).

This new parameter is disabled by default.

Test plan

End-to-end tests have been added to api/handlers_e2e_test.go.

[codecov]

Codecov Report

Attention: Patch coverage is 58.33333% with 10 lines in your changes missing coverage. Please review.

Project coverage is 65.01%. Comparing base (25c7ab7) to head (498035f).
Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
api/converter_utils.go	57.14%	4 Missing and 2 partials ⚠️
api/generated/v2/routes.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1647      +/-   ##
==========================================
- Coverage   69.44%   65.01%   -4.44%     
==========================================
  Files          38       40       +2     
  Lines        7386     8111     +725     
==========================================
+ Hits         5129     5273     +144     
- Misses       1839     2402     +563     
- Partials      418      436      +18     

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[urtho]

As discussed internally, this needs to make use of group index by conditionally removing LIMIT when filtering by group id.
There is no other easy way for Postgres query optimizer to consider indexes other than primary (round,intra).
Filtering by group_id generates small enough results so that getting rid of LIMIT is safe.

Query optimizer spends weeks (>2 seconds) on optimizing this query if TXN happens to be partitioned.
Fortunaltely not the case for default deployments.

CREATE INDEX txn_grp ON public.txn USING btree (((txn #>> '{txn,grp}'::text[]))) WHERE ((txn #>> '{txn,grp}'::text[]) IS NOT NULL);