Add support for building composes including particular Koji builds.
Pungi now supports including Koji builds defined by particular NVR
using the `pkgset_koji_builds` option. This PR adds support for this
new feature in ODCS:
- For `tag` source_type, additional `builds` list in REST API can be used
to define extra builds which will be considered for inclusion in a compose
on top of the builds comming from the Koji tag.
- New `build` source_type is introduced which can be used to build a compose
including just Koji builds from `builds` list without any Koji tag.