debuginfod: add metadata query by buildid

debuginfod: add metadata query by buildid

This patch extends current metadata by search by buildid.
It allows clients to query for all packages (with exact versions)
and files associated with a specific buildid.

I'd like to use the exact package versions during core dump
analysis to match them with already known issues/bugs,
to check that involved packages are reasonable up-to-date
and to identify other invalid setup, such as mixing packages
from different Linux distros.

Search by buildid prepares alternative db query, handling
of results incl. gathering results from federated servers
remains the same.
It does not search for source files or source file archives.

Signed-off-by: Josef Cejka <jcejka@suse.de>