Draft: Cargo2: report individual source ref provenance

Joshua Zivkovicrequested to merge
joshua.zivkovic/bst-plugins-experimental:JZ/cargo2-source-provenance into master

With the (soon to be) addition of https://github.com/apache/buildstream/pull/2098, sources can now manually provide source provenance information, for the sake of multi-source source plugins like cargo2.

This MR makes use of that new functionality by updating cargo2 to be able to consume updated element files and report the information back to the buildstream core.

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