More efficient mime type parsing in server SDKs; avoid mime crate

[david-perez]

We're currently creating one OnceCell per operation backing a static mime::Mime type for the operation's expected (if present) client Accept header value, only to then decompose it into type + subtype and perform a case-insensitive string comparison check against the actual incoming Accept header value.

It would be more efficient to directly compare &strs and avoid incurring the atomic read.

A simpler, more efficient, and type-safe approach would be to directly switch to something like mediatype, which is const-constructible and zero-copy. Note that mime is not actively maintained.

See #2607 (comment) and #2629, the PR where we started using OnceCells for this.

[hlbarber]

#2629 already uses const in the following cases https://github.com/awslabs/smithy-rs/pull/2629/files#diff-da9c47cae3f2f75fc01b3625ee430f56f2f495870a77302c3c148e98f7341bd4R198-R200.

The migration to a const constructable struct would follow the same pattern.

[82marbag]

hyperium/mime#153 proposes to solve these issues in mime and actively maintain the crate. We work with other of their crates, if it's worth seeing what the proposal will look like and possibly continue with mime