lib/repo: Add collection ID support to OstreeRepo

Add {get,set}_collection_id() methods to OstreeRepo and some documentation
about the concept of a collection ID which globally identifies an
upstream repository. See the documentation for more details.

This will be used in future commits. For now, the new API is marked as
experimental (--enable-experimental-api).

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #924
Approved by: cgwalters

apidoc/ostree-experimental-sections.txt

@@ -23,5 +23,7 @@ ostree_remote_get_name
 
 <SECTION>
 <FILE>ostree-misc-experimental</FILE>
+ostree_repo_get_collection_id
+ostree_repo_set_collection_id
 ostree_validate_collection_id
 </SECTION>