List /media endpoint

[oguzkocer]

This PR implements list /media endpoint by mostly following the established patterns. Here are a few things worth noting:

• 0b8b06c implements FromStr for PluginStatus. I was planning to work on /plugins pagination and later realized that the endpoint doesn't support pagination. Since this is a useful addition, I didn't want to throw it away and decided to include in this PR.
• MediaId was temporarily implemented in wp_api/src/posts.rs but now moved to wp_api/src/media.rs.
• A new JsonValue type is introduced. The media_details field in /media has a dynamic type without a clear set of fields. This new JsonValue type will be used in such cases. @jkmassel mentioned that he had to implement something similar while working in Add initial OAuth 2 support #287 and we agreed that this generic version is a reasonable addition.
• The PR implements both MediaType & MediaTypeParam types. This is because the server accepts values such as image, video, text, application, audio to filter the media list, but it returns image or file.
• There is likely an error in the status of /media documentation. The Schema section indicates status is One of: publish, future, draft, pending, private, however using status=publish will result in InvalidParam error saying status[0] is not one of inherit, private, and trash.. Our test site also returns inherit for every media object. That's why MediaStatus is only implemented to have inherit, private, trash variants - with the addition of Custom(String) variant for fallback.

[jkmassel]

Code looks pretty straightforward – I tried it out from Swift (as usual) and it worked great as usual.