Filter repositories by declared schema using repository properties

[thatbudakguy]

This updates the clone and pull tasks to only apply to repositories
that declare their conformance with a particular OGM schema version
(v1.0 or Aardvark).

This is implemented via a repository custom property, which is set
at the organization level and can be enabled on a per-repository
basis. Unlike repository topics, this value is a controlled vocabulary
and is unique to the OGM organization.

This functionality replaces the "denylist" that was a hardcoded list
of repositories that shouldn't be harvested (e.g. because they were
tools or didn't contain metadata). In this version, providers can
opt-in to harvesting by adding the relevant schema property.

After this change, you no longer need to clone repositories that
don't match the schema version you are using, which saves disk
space and processing time.

[thatbudakguy]

Here's the interface for setting a repository custom property: