Add encoding.TextUnmarshaler support for slice query parameters

[B94715]

I must apologise, I accidentally pushed a PR (#1020) mistakenly commenting out tests that I did not own and didn't catch this until it was too late. I closed the PR immediately to rectify the issue but wasn't aware that GitHub automatically locks closed PRs when --force-with-lease is applied to a git push. The original PR was thus

--

This directly addresses #488 and is an alternative to #998.

parseSliceInto() was missing the encoding.TextUnmarshaler fallback that parseInto() already has. This aims to address that.

[codecov]

Codecov Report

❌ Patch coverage is 96.15385% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.05%. Comparing base (0dc6f40) to head (b121ed1).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
huma.go	96.15%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1021      +/-   ##
==========================================
- Coverage   93.07%   93.05%   -0.03%     
==========================================
  Files          23       23              
  Lines        4781     4868      +87     
==========================================
+ Hits         4450     4530      +80     
- Misses        271      274       +3     
- Partials       60       64       +4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[wolveix]

Awesome work, thanks @B94715!