Make `Content-Type` check in `ServerHttpBoundProtocolGenerator` protocol-agnostic (#1605)

The `Content-Type` check of the incoming request in
`ServerHttpBoundProtocolGenerator` is performed by comparing against the
configured protocol directly within the class.

Delegating to the protocol enables the implementation of protocols in
decorators loaded from the classpath in other projects outside
smithy-rs.

This commit makes the `ServerHttpBoundProtocolGenerator`
protocol-agnostic by looking up the expected incoming request
`Content-Type` header from the protocol implementation directly. To this
end, the macro-generated `check_<protocol>_content_type` functions from
the `aws-smithy-http-server` runtime have been replaced with a single
general `check_content_type` function that takes in the expected mime
type.