main/file: Add the ability to play media in the media cache

This patch allows applications/APIs that access media through the core file
APIs to play media in the media cache. Prior to determining if a 'filename'
exists, the filename is passed to the media cache's retrieve API call. If
that call succeeds, the local file specified passed back by the API is
opened for streaming. When used in this fashion, the 'filename' is actually
a URI that the media cache process and understand.

ASTERISK-25654 #close

Change-Id: I73b6e2e90c3e91b8500581c45cdf9c0dc785f5f0