fix: consumer group consumer create request body to align with API spec

[harshadixit12]

This PR fixes the request body sent while creating a consumer group consumer - we were sending an array of UUIDs in consumer field while according to API spec we should be sending a single UUID.
Fixes https://konghq.atlassian.net/browse/APIOPS-121

API spec for reference: https://docs.konghq.com/gateway/api/admin-ee/latest/#/operations/post-consumer_groups-group_name_or_id-consumers_with_workspace

Haven't added additional tests since there is already one - https://github.com/Kong/go-kong/blob/main/kong/consumer_group_consumer_test.go#L36
I have tested this on deck through integration and manual tests.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 51.72%. Comparing base (da4e510) to head (8ff1ae3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #545      +/-   ##
==========================================
- Coverage   51.74%   51.72%   -0.02%     
==========================================
  Files          74       74              
  Lines        6196     6194       -2     
==========================================
- Hits         3206     3204       -2     
  Misses       2372     2372              
  Partials      618      618              

Flag	Coverage Δ
3.4	49.58% <100.00%> (-0.02%)	⬇️
3.7	48.43% <100.00%> (-0.02%)	⬇️
3.8	49.90% <100.00%> (-0.02%)	⬇️
3.9	50.00% <100.00%> (-0.02%)	⬇️
community	38.42% <0.00%> (+0.01%)	⬆️
enterprise	50.43% <100.00%> (-0.02%)	⬇️
integration	51.72% <100.00%> (-0.02%)	⬇️

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