fix: correct inconsistency for multiple consumers with custom_id fields

[GGabriele]

This commit corrects a bug preventing consumers with equal Username and CustomID to always be correctly handled by decK.

For example, syncing the following state file works as expected:

$ cat kong.yaml
_format_version: "3.0"
consumers:
- username: TestUser
- username: OtherUser
  custom_id: TestUser

$ deck sync
creating consumer OtherUser
creating consumer TestUser
Summary:
  Created: 2
  Updated: 0
  Deleted: 0

But syncing the following state file (same content as the first one, but with consumers in reverse order) doesn't work:

_format_version: "3.0"
consumers:
- username: OtherUser
  custom_id: TestUser
- username: TestUser

$ deck sync
Error: building state: inserting consumer TestUser: entity already exists

This commit is making sure this bogus inconsistency is corrected.

[codecov-commenter]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (c155000) 33.65% compared to head (b43cb30) 33.64%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1037      +/-   ##
==========================================
- Coverage   33.65%   33.64%   -0.01%     
==========================================
  Files         101      101              
  Lines       12409    12408       -1     
==========================================
- Hits         4176     4175       -1     
- Misses       7824     7826       +2     
+ Partials      409      407       -2     

Files	Coverage Δ
state/consumer.go	71.55% <14.28%> (-0.25%)	⬇️

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