Body media type in HTTP malformed request tests

[david-perez]

Both httpRequestTests and httpResponseTests have a bodyMediaType field. This field allows test generators, among other things, to know how the body field is encoded.

Because the body parameter is a string, binary data MUST be represented in body by base64 encoding the data (for example, use "Zm9vCg==" and not "foo")

However, HttpMalformedRequestDefinition does not have a bodyMediaType field. So far in smithy-rs we have sent these malformed requests by sending the body contents as-is, and this has worked well because all protocols that Smithy currently supports use text-based serialization formats. However, with the advent of Smithy RPC v2 CBOR, this is no longer case, and so we need a way to tell the test generator whether to base64-decode body or not before sending the request: the bodyMediaType field would fulfill this need.

Can we add bodyMediaType to HttpMalformedRequestDefinition?

[mtdowling]

Sounds good to me.