… LB policies (#32709)

This paves the way for making pick_first the universal leaf policy (see
#32692), which will be needed for the dualstack design. That change will
require changing pick_first to see both the raw connectivity state and
the health-checking connectivity state of a subchannel, so that we can
enable health checking when pick_first is used underneath round_robin
without actually changing the pick_first connectivity logic (currently,
pick_first always disables health checking). To make it possible to do
that, this PR moves the health checking code out of the subchannel and
into a separate API using the same data-watcher mechanism that was added
for ORCA OOB calls.

…directly (grpc#32692)

More work on the dualstack backend design:
- Change round_robin to delegate to pick_first instead of creating
subchannels directly.
- Change pick_first such that when it is the child of a petiole policy,
it will unconditionally start a health watch.
- Change the client-side health checking code such that if client-side
health checking is not enabled, it will return the subchannel's raw
connectivity state.
- As part of this, we introduce a new endpoint_list library to be used
by petiole policies, which is intended to replace the existing
subchannel_list library. The only policy that will still directly
interact with subchannels is pick_first, so the relevant parts of the
subchannel_list functionality have been copied directly into that
policy. The subchannel_list library will be removed after all petiole
policies are updated to delegate to pick_first.

…hannels directly (grpc#32692)"

This reverts commit 27a778f.

This reverts the following PRs: #32692 #33087 #33093 #33427 #33568

These changes seem to have introduced some flaky crashes. Reverting
while I investigate.

This reverts the following PRs: grpc#32692 grpc#33087 grpc#33093 grpc#33427 grpc#33568

These changes seem to have introduced some flaky crashes. Reverting
while I investigate.

This rolls forward only the pick_first changes from #32692, which were
rolled back in #33718. Specifically:
- Changes PF to use its own subchannel list implementation instead of
using the subchannel_list library, since the latter will be going away
with the dualstack changes.
- As a result of no longer using the subchannel_list library, PF no
longer needs to set the `GRPC_ARG_INHIBIT_HEALTH_CHECKING` channel arg.
- Adds an option to start a health watch on the chosen subchannel, to be
used in the future when pick_first is the child of a petiole policy.
(Currently, this code is not actually called anywhere.)

…34222)

Rolls forward part of the dualstack changes, mostly from #33427 and a
little bit from #32692, both of which were reverted in #33718.
Specifically:
- For petiole policies, unconditionally start health watch on
subchannels, even if client side health checking is not enabled; in this
case, the health watch will report the subchannel's raw connectivity
state.
- Fix edge cases in health check reporting that occur when a watcher is
started before the initial state is reported.
- When client-side health checking fails, add the subchannel's address
to the RPC failure status message.
- Outlier detection now works only via the health checking watch, not
via the raw connectivity state watch.
- Remove now-unnecessary hack to ensure that outlier detection does not
work for pick_first.

Rolls forward the remaining changes from #32692, which were rolled back
in #33718.