Replace public use of mime crate with &str

[nashley]

This PR introduces a breaking change.

Motivation

Resolves #637.
Removes exposure of the mime crate in the public library.
This will allow users to handle the underlying &str instead and optionally parse Content-Type headers with their library of choice (if desired).
This also allows axum to remove the dependency on mime in the future without requiring another breaking change.

Solution

&str has replaced &mime::Mime as axum::extract::multipart::Field::content_type's return type

Searching docs for mime shows no results:

> cd axum/src/docs
> rg mime
> # (no results)

Questions

• Does this PR need to be against a different branch other than main for the 0.5 release?
• Should there be more tests? I couldn't find a good way to create parameterized tests without adding another dependency or creating a potentially-esoteric macro.
• Should there be a doc test for content_type()? Seems like it'd either be excessive or incomplete.

[davidpdrsn]

Does this PR need to be against a different branch other than main for the 0.5 release?

I'm gonna make a branch for 0.5 soon and then we can merge this into that.

Should there be more tests? I couldn't find a good way to create parameterized tests without adding another dependency or creating a potentially-esoteric macro.

Thank you for adding a test! I dont think we need to add more tests here 😅

Should there be a doc test for content_type()? Seems like it'd either be excessive or incomplete.

Nah I think it's fine as is.