add ostree plugin

Note: The CI for this branch has been configured so that it can make use of buildstream!1261 (merged) prior to it landing. This is not it's final state. This quick hack demonstrates the ostree's tests running and passing, but breaks the rest of the tests as it's using a buildstream testsuite image for now. Once the above mentioned MR has made it into the buildstream:latest docker image, this can go back to using that image and all the tests should pass.

Was originally raised as bst-plugins-ostree!1 (closed), but it was decided that it would be more sensible to move this here until the proposal here is implemented.