chore: to_lowercase -> to_lowercase_cow

[minht11]

Summary

Follow up to #4014
Implement to_lowercase_cow and use it instead to_lowercase. Enforce future usage with clippy rule.
There are still some to_lowercase left which are trickier to migrate, those and the clippy enforcement will be done with separate PR.

Test Plan

All tests should pass.

[dyc3]

Make sure to add update the clippy lint in clippy.toml too

[minht11]

@dyc3 There are still places which are cannot easily migrated to Cow version. For now I just want to see if there is any perf improvement.
I will likely finish migrating/enforcing with separate PR.

[minht11]

I am not sure what todo here. OsStr does not support to_lowercase. Implementing it manually is hard and we don't really need it.

Is leaving as is fine? Do we need separate trait?

[dyc3]

I suppose a separate trait would be safest.

[minht11]

I split it into two separate traits, shame trait-alias are not stable so only one trait could be imported for both. We could use trait-set macro but I imagine introducing new crate for this would be overkill.

[codspeed-hq]

CodSpeed Performance Report

Merging #4030 will degrade performances by 7.41%

_{Comparing minht11:chore/replace-to-lowercase-with-cow (fdec823) with main (d20ad1e)}

Summary

❌ 1 regressions
✅ 106 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	minht11:chore/replace-to-lowercase-with-cow	Change
❌	router_17129688031671448157.ts[cached]	2 ms	2.2 ms	-7.41%