Anti-Affinity groups should allow memberships consisting of affinity groups

[smklein]

Related to #1705

The first iteration of Affinity/Anti-Affinity groups only uses a membership type of "instance UUID".

This allows clients to make individual instances affine or anti-affine from one another, but it isn't yet possible to describe "I want this group of instances to be anti-affine from one another".

This feature was described in RFD 522:

An Anti-Affinity Group is a collection of Interfaces or Affinity Groups that are intended to be deployed in different fault domains

This issue tracks that feature. This should be possible by adding a new anti-affinity member type.

Note that this feature is asymmetric with respect to affinity groups. There is currently no intention to allow "affinity groups consisting of anti-affinity groups" (since that construction wouldn't make much sense).

[smklein]

This was attempted in #7572, and is discussed at length in https://github.com/oxidecomputer/rfd/pull/869. We are deferring it for now.

#7572 provides an example of how this feature might be implemented, but we are punting on it for the reasons mentioned in RFD 522