Add new lint unnecessary_dedup_by

[moses7054]

This PR adds a new lint, unnecessary_dedup_by, which suggests using Vec::dedup_by_key instead of Vec::dedup_by,
when the comparison is equivalent to a simple key extraction.

The implementation is adapted from unnecessary_sort_by and copies the same helper logic.

changelog: [unnecessary_dedup_by]: add new lint

fixes #16481

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[github-actions]

Lintcheck changes for a7f59a8

Lint	Added	Removed	Changed
clippy::unnecessary_dedup_by	1	0	0

This comment will be updated if you push new changes

[ada4a]

It's a bit concerning that the lint doesn't actually reuse the logic, but rather copy-pastes it...

But more generally, I think this would warrant creating a more general lint that looks at every instance of "x_by could be replaced with x_by_key" -- maybe call it needless_x_by or manual_by_key

[moses7054]

@ada4a yeah, my initial intent with the copy-paste was to keep the lint self-contained and avoid a hard dependency on unnecessary_sort_by, so that all logic related to this lint lives in one place.

I thought of extracting the common logic into a common utility and make a generic lint( as mentioned in the issue's description) but I am just starting to contribute to clippy 😅 so thought of keeping it simple and see what the maintainers say.
I am happy to modify the pr as required.

I changed the wording to "copy" in the pr description so that it will be more clear.