Refine definition of feed

[SteveLasker]

A feed is a great base for how we can create a series of statements for different artifacts, getting freshness for a receipt/or VEX report.
The current definition likely needs to expand a bit to account for:

• What are the versions of a specific artifact
• What are all the statements for a version of an artifact
• What is the latest statement for a specific contentType of a specific versioned artifact: (eg: what's the latest VEX for the net-monitor:v1 software?
• If the contentType is a referenced statement by reference, which stores SBOMs, VEX, Scan Reports, how do we drill into each if they all use the same payload contentType of satementByReference?

[yogeshbdeshpande]

@SteveLasker To move this to new repo with correct definition of the problem statement

[SteveLasker]

Closing as this has been copied over to: ietf-wg-scitt/draft-ietf-scitt-architecture#11