fix: added new method to fetch a consumer-group without consumers.

[Prashansa-K]

Reverted the existing Get method to keep listing consumers and added a new one that uses the query
parameter list_consumers=false to skip listing
consumers for performance reasons.
This is done so as to fix dump errors coming in
deck when using gw 3.9+ where consumer-groups
do not show the associated consumers. We will
push the performant option behind a flag.

For: Kong/deck#1483

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 33.33333% with 10 lines in your changes missing coverage. Please review.

Project coverage is 55.01%. Comparing base (0008da3) to head (21a3423).

Files with missing lines	Patch %	Lines
kong/consumer_group_service.go	33.33%	8 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #494      +/-   ##
==========================================
- Coverage   55.16%   55.01%   -0.15%     
==========================================
  Files          72       72              
  Lines        5754     5769      +15     
==========================================
  Hits         3174     3174              
- Misses       1954     1969      +15     
  Partials      626      626              

Flag	Coverage Δ
2.1	32.95% <0.00%> (+0.01%)	⬆️
2.2	43.28% <0.00%> (-0.01%)	⬇️
2.3	43.83% <0.00%> (-0.02%)	⬇️
2.4	43.90% <0.00%> (-0.02%)	⬇️
2.5	43.90% <0.00%> (-0.02%)	⬇️
2.6	43.90% <0.00%> (-0.02%)	⬇️
2.7	45.25% <33.33%> (-0.12%)	⬇️
2.8	45.36% <33.33%> (-0.02%)	⬇️
3.0	48.86% <33.33%> (+0.63%)	⬆️
3.1	50.18% <33.33%> (+0.63%)	⬆️
3.2	50.18% <33.33%> (+0.63%)	⬆️
3.3	50.18% <33.33%> (+0.63%)	⬆️
3.4	52.17% <33.33%> (+0.62%)	⬆️
3.5	50.35% <33.33%> (+0.63%)	⬆️
3.6	50.94% <33.33%> (-0.14%)	⬇️
3.7	50.94% <33.33%> (+0.63%)	⬆️
3.8	52.52% <33.33%> (+0.52%)	⬆️
community	41.06% <0.00%> (-0.11%)	⬇️
enterprise	53.63% <33.33%> (-0.14%)	⬇️
integration	55.01% <33.33%> (-0.15%)	⬇️

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[Prashansa-K]

@pmalek Could you please help with this PR review?
I need to unblock the use of deck gateway dump for GW 3.9+