Content repository: No API for finding info on a document (SPEC-448)

There's no API for a client to request information from the HS on a document prior to attempting to download it. While the event may optionally have some information on an included file/image it's odd not to be able to query it from the repository.

Things I might want to know before attempting a download:
a) Size in bytes (before asking for a ziggabyte file that I'll discard part way through)
b) content type
for images:
c) width/height
? d) what size thumbnails (if any) the server would prefer us to ask for; e.g. a list of cached sizes it has.

(Imported from https://matrix.org/jira/browse/SPEC-448)

(Reported by Dave Gilbert)

Start by reviewing the Matrix protocol specification and the content repository API described in this issue. Define the metadata request and response needed for document size, content type, image dimensions, and available thumbnail sizes; done means the specification clearly describes a usable API for these queries.

Written by the indexing model from the issue text.