[chore] Make mapstructure hooks safe against untyped nils

[mx-psi]

Description

If we enable DecodeNil as true, we may have untyped nils being passed. Unfortunately, these are not valid values for reflect, which leads to surprising behavior such as golang/go/issues/51649.

Unfortunately, the default hooks from mapstructure do not deal with this properly. To account for this, we:

• Vendor and change the ComposeDecodeHookFunc function so that this case is accounted for the kinds of hooks that just won't work with untyped nils
• Create a safe wrapper for the hooks that do work with untyped nils. This wrapper is used in all hooks, but in the interest of keeping as close to what I would imagine upstream will accept, I did not add this to the compose function.

This should not have any end-user observable behavior.

Link to tracking issue

Attempt to work around #12996 (comment)

Working towards #12981

[codecov]

Codecov Report

Attention: Patch coverage is 76.00000% with 18 lines in your changes missing coverage. Please review.

Project coverage is 91.62%. Comparing base (6bd77b3) to head (94dd45c).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...p/internal/third_party/composehook/compose_hook.go	76.19%	14 Missing and 1 partial ⚠️
confmap/confmap.go	75.00%	2 Missing and 1 partial ⚠️

❌ Your patch status has failed because the patch coverage (76.00%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13001      +/-   ##
==========================================
- Coverage   91.67%   91.62%   -0.05%     
==========================================
  Files         503      504       +1     
  Lines       27819    27888      +69     
==========================================
+ Hits        25502    25553      +51     
- Misses       1828     1844      +16     
- Partials      489      491       +2     

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[evan-bradley]

Is there a good way to test the added functionality in this PR, or does it just feed into #12996 and we can test it there?

[mx-psi]

Is there a good way to test the added functionality in this PR, or does it just feed into #12996 and we can test it there?

I think this should not make any difference, I was relying on the existing test suite, including the tests I added on #12998. We will see a behavioral difference with #12996 and we can test it further there