cherrypick-2.0: config/storage: per-replica constraints, improved locality handling

[a-robinson]

This creates a new allowable format for zone config constraints, which
lets the user apply different sets of constraints to different numbers
of replicas within a zone. See the included tests for what the new format
looks like.

Fixes #19985

It also improves the handling of localities that are more or less full
than one another for some reason outside the system's control (e.g. if
one has more nodes than the others or if some constraints have required
more ranges to be in one). In such cases, this does a better job of
balancing ranges amongst the nodes that are more or less full because we
compare them amongst each other now rather than comparing them to the
StoreList averages of all the stores in the cluster.

Fixes #20751

This also deprecates positive (non-required, non-prohibited) constraints.
New positive constraints cannot be set, and existing positive
constraints will be ignored.

Release note (cli change): Replication zone constraints can now be
specified on a per-replica basis, meaning you can configure some
replicas in a zone's ranges to follow one set of constraints and other
replicas to follow other constraints.

Release note (backwards-incompatible change): Positive constraints in
replication zone configs no longer work. They have never been
documented or officially supported, but will no longer be allowed at
all. Any existing positive constraints will be ignored.

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Cherry-picks #22412/#22819 into the release-2.0 branch.

@cockroachdb/release

[cockroach-teamcity]

This change is 

[a-robinson]

Merging to make sure this gets into the alpha if the SHA is picked tonight, on the understanding that putting this in 2.0 is the agreed-upon plan and all the code was reviewed in #22412/#22819. There were no conflicts when cherry-picking.