Devfile 2.1 support

[scottkurz]

Implement the design for OpenLiberty/open-liberty#19019.

One caveat: Though we call this the "devfile 2.1 support" feature.... I don't think there's a particular need to stop short of moving to the 2.2-schema level and starting to add some of the 2.2 level features... nor is there a need to create branches for this 2.1 effort.

This is a checkpoint with respect to what we socialized in the design, but we can continue to work ahead in this single stream.