Enhance request pattern matching

[lundberg]

To match and mock a request with certain content, e.g. posted data, a request callback is currently needed.

It would be nice to have an alternative way in the api, like http method and url, to allow request content matching, as discussed and proposed in #83.

The HTTPX request API also supports cookies and files, which would be of interest to match on, as mentioned in #79.

• cookies (investigate format), Fixed by More patterns and lookups #99.
• content, Fixed by Implement content, data and json patterns #106
• data, Fixed by Implement content, data and json patterns #106
• json, Fixed by Implement content, data and json patterns #106
• files (investigate POST fields) Moved to Add a files pattern? #115