Bump `lambda_http` dependency of `aws-smithy-http-server` to 0.8.0 (#2685)

This patch also removes the unneeded dependency on `lambda_runtime` by
`aws-smithy-http-server-python`.

This patch also refactors `LambdaHandler`'s `convert_event` to avoid
cloning the URI path when not needed.

This is a breaking change. See #2676 why.

This patch also bumps `aws-smithy-http-server` dependency on `mime` to
0.3.4.

`cargo +nightly-2022-11-16 minimal-versions check --all-features`
otherwise
fails when using 0.3.0, because we require `impl fmt::Display for
mime::FromStrError`, which was first introduced in 0.3.4.

As to why `minimal-versions` is now picking `mime` 0.3.0 with this
patch, it's
because in `lambda_http` 0.7.3, they had [`mime =

"0.3.16"`](https://github.com/awslabs/aws-lambda-rust-runtime/blob/99dba6447253ac87cf3cefeb2ba130b50514f9df/lambda-http/Cargo.toml#L35-L35),
and in `lambda_http` 0.8.0, they've now relaxed that to [`mime =

"0.3"`](https://github.com/awslabs/aws-lambda-rust-runtime/blob/393d6447bea0502e1f939d197f4facc228e6e007/lambda-http/Cargo.toml#L36).


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

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