Fully Implement gating Type for Use By All Provider Types

[lb4368]

Problem description
In #564 work will be done to create a normalized phase plan for all provider types in the gating type. Need to build out the rest of the type so that common entrypoints and manifests are managed at a common type level.

Proposed change
Make the gating type a fully-fledged type by moving the meat of the phase payload entrypoints from the site level to the type level. The site-level phase payload entrypoints will be very skinny after this (just like in treasuremap), making only the tweaks that are needed to tune the phase payloads per-provider.

[matthew-fuller]

Please assign this to me.

[matthew-fuller]

https://review.opendev.org/c/airship/airshipctl/+/799902