cmd/msrfetch: fetch from enriched composite data

This creates a content-addressed store for all media files. It is not
yet usable but an API and cdn backend will be implemented from it in a
later commit, as well as tooling for export and tagging.

Signed-off-by: Yonah <contrib@gensokyo.uk>

.gitignore

@@ -7,6 +7,9 @@
 *.pkg
 /msrfetch
 
+# Content-addressed media files
+/data
+
 # Test binary, built with `go test -c`
 *.test