fix: Post selector with Go templates in ApplicationSet

[m13t]

This PR fixes #12524 by allow users to reference nested properties when using Go template syntax in application sets. Previously only top-level string keys could be used as post selectors. When matching labels, this PR will now allow referencing nested keys by concatenating each key with a dot (.).

The original match values tests have been duplicated with Go templates enabled to provide more extensive tests.

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[codecov]

Codecov Report

Patch coverage: 47.36% and project coverage change: -0.02 ⚠️

Comparison is base (0a8a71e) 49.17% compared to head (e90a476) 49.16%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #13584      +/-   ##
==========================================
- Coverage   49.17%   49.16%   -0.02%     
==========================================
  Files         248      248              
  Lines       42872    42881       +9     
==========================================
- Hits        21084    21083       -1     
- Misses      19687    19695       +8     
- Partials     2101     2103       +2     

Impacted Files	Coverage Δ
...licationset/generators/generator_spec_processor.go	59.18% <47.36%> (-7.11%)	⬇️

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[m13t]

I've made a slight addition to my original commit for this to create a generic map flattening function as this would also be useful for supporting nested keys in the merge generator mergeKeys as this also has issues when using Go templates. Assuming this PR is okay I'll look to tackle that as a separate PR.

[crenshaw-dev]

Thanks so much for this PR! And yes, fixing mergeKeys would also be huge.

[m13t]

@crenshaw-dev I've rebased this PR and there looks to be some issues with the recently merged in test changes you did on the matrix generator as part of m13t@fafee48.

Can't understand why the changes in the PR you merged worked but do not in my branch. Seems as though the non-go template variables are not being interpolated. Any ideas?

[m13t]

@crenshaw-dev rebased and all working again now. This is good to go.

[crenshaw-dev]

Beautiful, thanks @m13t!

[crenshaw-dev]

/cherry-pick release-2.7

[gcp-cherry-pick-bot]

Cherry-pick failed with Merge error 74839c88214605918e9fc47a63ff9fdfd3651aa0 into temp-cherry-pick-022040-release-2.7

[crenshaw-dev]

/cherry-pick release-2.6

[gcp-cherry-pick-bot]

Cherry-pick failed with Merge error 74839c88214605918e9fc47a63ff9fdfd3651aa0 into temp-cherry-pick-022040-release-2.6

[crenshaw-dev]

/cherry-pick release-2.5

[gcp-cherry-pick-bot]

Cherry-pick failed with Merge error 74839c88214605918e9fc47a63ff9fdfd3651aa0 into temp-cherry-pick-022040-release-2.5

[crenshaw-dev]

@m13t would you mind opening PRs against release-2.5, release-2.6, and release-2.7? Cherry-pick bot is finicky.

[m13t]

@crenshaw-dev manual cherry-picked PRs are in