This is a prerelease of v0.4.0, cut from the next branch. v0.4.0 will follow when some next-only features are merged into master.
- Removes DSSE code and moves it to https://github.com/secure-systems-lab/go-securesystemslib (@shibumi)
- Adds SPIFFE integration (@pxp928, @mikhailswift)
- Re-organizes SLSA provenance formats based on versioning, adds v0.2 (@priyawadhwa)