Update 301-303 to add provenance of bundles

[trishankatdatadog]

• [x] Add extension of MVP (TUF+in-toto) in 301 (metadata repos)
• [x] Discuss extension in 302 (signing)
• [x] Discuss extension in 303 (verification)

[radu-matei]

@trishankatdatadog - this looks great! Before merging, could you please rebase this against master?

Thanks!

[trishankatdatadog]

Done, thanks! @radu-matei