Add a rule to check for valid / recommended media types

[mikekistler]

Most Azure services simply accept and return "application/json" bodies, and this is the recommended design (except for PATCH, which should be "application/merge-patch+json".

A few other media types are common and generally acceptable:

• "application/octet-stream", "text/plain", "text/csv"
• "multipart/form-data" when used alone (see below)

But there are cases where services define invalid content types or use valid types in a way that is not recommended:

• "" can only be used for the type or subtype -- not as a wildcard character. So "application/+json" is invalid (according to the BNF in RFC 6838) -- this comes from appconfiguration.json.
• "multipart/form-data" with any other content type. This is a problem because OAS2 does not allow both "form" and "body" parameters on a single operation.
• "text/json" -- commonly seen with "application/json". These are essentially the same thing, so no need to have both of them, especially in "produces"

We may want to decide on whether parameters should be allowed -- there are more than a few instances of ""application/json; charset=utf-8" in the current specs.