Lazy initialize mime accept header (#2629)

## Description

Do not parse and initialize the mime, known at compile time, on every
request.

See
https://github.com/awslabs/smithy-rs/pull/2607#discussion_r1172607704




## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

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choice._

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Signed-off-by: Daniele Ahmed <ahmeddan@amazon.de>