Clarify specification on Bundle and Envelope media types

[marcelamelara]

This PR updates the specs for Bundles and Envelopes to more formally describe conventions around attestation layer media types when these are stored in arbitrary storage systems, per #271 (comment)

Fixes #271

[joshuagl]

I agree that this PR improves clarity, thanks for working on it.

I'm worried we're not consistent in using capitalised names for components in the attestation model (Bundle, Attestation, Envelope, Statement, Predicate, etc). Should we do a consistency pass and create some contributor documentation explaining conventions like this?

[marcelamelara]

I'm worried we're not consistent in using capitalised names for components in the attestation model (Bundle, Attestation, Envelope, Statement, Predicate, etc). Should we do a consistency pass and create some contributor documentation explaining conventions like this?

Agreed, I don't think we're being consistent on this right now. Personally, I find it helpful to capitalize these terms denote when we're talking about attestation model components vs. not, but I wasn't sure if everyone would agree. If we do, then we should definitely follow your suggestion of doing a consistency pass and adding documentation for this.

[adityasaky]

Some minor comments / updates aside, this LGTM.

[TomHennen]

This looks great!

[trishankatdatadog]

LGTM, thanks

[joshuagl]

This is excellent, thank you!