Improve external CI pipeline support
Background:
Currently the support provided by API for external builders is very limited. You can only report commit status and hope it works. There is no way to specify that multiple builders are part of same build, or specify stages they belong to. Some of us are not able to use Gitlab CI for various reasons, one of those being that we cannot have in-tree gitlab config files.
Would be really great if you could provide better support for external builders, such as ability to specify stages, and also ability to mark builds to same pipeline reliably.