grpc-js: Return LB policy configs from resolvers in JSON form

[murgatroid99]

More generally, the principle here is that LB policy configs should be created and passed around as plain objects until they need to be passed to an LB policy, at which time they are parsed into a typed object.

In addition, an LB policy config for a non-leaf/petiole policy has a list of possible configs for each child policy, and the first usable config in the list is used. Prior to this change, that selection took place at child policy creation time. Now, it takes place at LB policy config parse time, recursively.

The purpose of this PR is to make it easier to implement custom LB policies. Specifically, that spec requires generating arbitrary LB policy configs from Struct messages.

API changes:

• The interface ServiceConfig and interfaces for its members MethodConfig, LoadBalancingConfig, and RetryPolicy have been moved out of the experimental namespace, into the stable public API.
• The LoadBalancingConfig type now refers to simple raw objects instead of class instances.
• experimental.getFirstUsableConfig, experimental.validateLoadBalancingConfig, and experimental.OutlierDetectionLoadBalancingConfig have been removed.
• experimental.selectLbConfigFromList, experimental.parseLoadBalancingConfig, and experimental.OutlierDetectionRawConfig have been added.

[gnossen]

TIL the word petiole.