have bst-track be able to update tar sources
Background
Currently bst-track is most useful for git and git_tag sources, which allows implementation of updaters such as https://gitlab.com/BuildStream/infrastructure/gitlab-merge-request-generator
In libreML and freedesktop-sdk there are a number of elements that have tarball sources, that bst-track does not currently work on. It would be good to be able to update those sources.