Changed trailing slash redirection behaviour

[Atennop1]

Now setting RedirectTrailingSlash to false will result in treating /foo and /foo/ as the same endpoints and won't cause redirection, they will share common handlers instead.

The current implementation is inefficient because I trigger second search if first failed, but I didn't figure out how to pass this information to findCaseInsensitivePathRec without breaking everything (and didn't know would it be right, how do you think?). Also, I would like to replace RedirectTrailingSlash with some TrailingSlashBehaviour enum, but it will break backward compatibility.

This pull request is meant to solve #4411

Would like to receive critics :)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.99%. Comparing base (3dc1cd6) to head (18d33bd).
⚠️ Report is 230 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4499      +/-   ##
==========================================
- Coverage   99.21%   98.99%   -0.22%     
==========================================
  Files          42       44       +2     
  Lines        3182     2999     -183     
==========================================
- Hits         3157     2969     -188     
- Misses         17       21       +4     
- Partials        8        9       +1     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.99% <100.00%> (?)
-tags go_json	98.92% <100.00%> (?)
-tags nomsgpack	98.98% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.24	98.99% <100.00%> (?)
go-1.25	98.99% <100.00%> (?)
macos-latest	98.99% <100.00%> (-0.22%)	⬇️
ubuntu-latest	98.99% <100.00%> (-0.22%)	⬇️

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