release-23.1: roachtest: add failover variants for partial network partitions

[erikgrinaker]

Backport 5/5 commits from #103254.

/cc @cockroachdb/release

Release justification: test-only change.

---

roachtest: improve zone config readability in failover tests

This patch improves readability of zone configs in failover tests, by tweaking the APIs. It also fixes a bug which placed replicas on the wrong nodes in failover/partial/lease-liveness.

roachtest: improve failover/partial/lease-liveness

This test did not sufficiently constrain range/lease placement, which caused occasional permanent unavailability as it randomly hit other failure modes than the one it's trying to test.

This patch makes the test more prescriptive, by separating out the system ranges, SQL gateways, liveness leaseholder, and user ranges, and only introducing a partial partition between a user leaseholder and the liveness leaseholder.

roachtest: add failover/partial/lease-gateway

This patch adds a roachtest that benchmarks the pMax unavailability during a partial network partition between a SQL gateway and a leaseholder. We currently don't handle this failure mode at all, and expect this to result in permanent unavailability.

kvserver: add COCKROACH_DISABLE_LEADER_FOLLOWS_LEASEHOLDER

This patch adds COCKROACH_DISABLE_LEADER_FOLLOWS_LEASEHOLDER which will disable colocation of the Raft leader and leaseholder. This is useful for tests.

roachtest: add failover/partial/lease-leader

This patch adds a roachtest that benchmarks unavailability during a partial partition between a Raft leader and leaseholder.

Resolves #94614.
Touches #93503.
Epic: none

Release note: None

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is