Code-generate const Mime values for each expected Content-Type in a request

[david-perez]

Currently, we re-parse expected request Content-Type header values into mime::Mimes every time we service a request. It'd be more efficient if aws_smithy_http_server::protocol::content_type_header_classifier_smithy took in directly a const Mime type.

Note that we already do this to check the Accept header (against the Content-Type of the response):

smithy-rs/codegen-server/src/main/kotlin/software/amazon/smithy/rust/codegen/server/smithy/protocols/ServerHttpBoundProtocolGenerator.kt

Lines 243 to 261 in e912cf5

	val verifyAcceptHeaderStaticContentTypeInit =
	writable {
	httpBindingResolver.responseContentType(operationShape)?.also { contentType ->
	val init =
	when (contentType) {
	"application/json" -> "const $staticContentType: #{Mime}::Mime = #{Mime}::APPLICATION_JSON;"
	"application/octet-stream" -> "const $staticContentType: #{Mime}::Mime = #{Mime}::APPLICATION_OCTET_STREAM;"
	"application/x-www-form-urlencoded" -> "const $staticContentType: #{Mime}::Mime = #{Mime}::APPLICATION_WWW_FORM_URLENCODED;"
	else ->
	"""
	static $staticContentType: #{OnceCell}::sync::Lazy<#{Mime}::Mime> = #{OnceCell}::sync::Lazy::new(|| {
	${contentType.dq()}.parse::<#{Mime}::Mime>().expect("BUG: MIME parsing failed, content_type is not valid")
	});
	"""
	}
	rustTemplate(init, *codegenScope)
	}
	}

It's probably better if we decide on #2666 before tackling this issue, to avoid throwaway work.

Originally source of this issue is #3690 (comment).