Enables LOAD_BALANCING_POLICY_CONFIG enum value in LbPolicy and supports
typed load balancers specified in load_balancing_policy. Continues work
done by Charlie Getzen <charliegetzenlc@gmail.com> in PR envoyproxy#15827.

Custom load balancers specified by load_balancing_policy are created as
implementations of ThreadAwareLoadBalancer. Thread-local load balancers
can be implemented as thread-aware load balancers that contain no logic
at the thread-aware level, i.e. the purpose of the thread-aware LB is
solely to contain the factory used to instantiate the thread-local LBs.
(In the future it might be appropriate to provide a construct that
abstracts away thread-aware aspects of ThreadAwareLoadBalancer for LBs
that don't need to be thread-aware.)

A cluster that uses LOAD_BALANCING_POLICY_CONFIG may not also set a
subset LB configuration. If the load balancer type makes use of
subsetting, it should include a subset configuration in its own
configuration message. Future work on load balancing extensions should
include moving the subset LB to use load balancing extensions.

Similarly, a cluster that uses LOAD_BALANCING_POLICY_CONFIG may not set
the CommonLbConfig, and it is not passed into load balancer creation
(mostly owing to its dubious applicability as a top level configuration
message to hierarchical load balancing policy). If the load balancer type
makes use of the CommonLbConfig, it should include a CommonLbConfig in
the configuration message for the load balancing policy.

Considerations for migration of existing load balancers:

- pieces of the ThreadAwareLoadBalancerBase implementation are specific
to the built-in hashing load balancers and should be moved into a base
class specifically for hashing load balancers. As it stands, custom load
balancing policies are required to implement a createLoadBalancer()
method even if the architecture of the LB policy does not require a
hashing load balancer. I think we would also benefit from disentangling
ThreadAwareLoadBalancerBase from LoadBalancerBase, as the former never
actually does any host picking.

- as we convert existing thread-local load balancers to thread-aware
load balancers, new local LBs will be re-created upon membership
changes. We should provide a mechanism allowing load balancers to
control whether this rebuild should occur, e.g. a callback that calls
create() for thread-aware load balancers by default, which can be
overridden to do nothing for thread-local LBs.

Signed-off-by: Eugene Chan <eugenechan@google.com>

Signed-off-by: Eugene Chan <eugenechan@google.com>

Signed-off-by: Eugene Chan <eugenechan@google.com>

Signed-off-by: Eugene Chan <eugenechan@google.com>

Removes LoadBalancingPolicy.Policy type from v4alpha. Changes the type of an unimplemented field.

Signed-off-by: Eugene Chan <eugenechan@google.com>

Signed-off-by: Eugene Chan <eugenechan@google.com>

Signed-off-by: Eugene Chan <eugenechan@google.com>

Signed-off-by: Eugene Chan <eugenechan@google.com>

Signed-off-by: Eugene Chan <eugenechan@google.com>