xds: use separate LB configs for EDS policy running with different code paths

[voidzcy]

In the latest design, the LB config for EDS policy started to diverge for the full xDS flow and for EDS only flow (internal. legacy?).

This changes separates the usage of LB configs for these two code paths:

For full xDS flow (received from CDS LB policy):

class EdsConfig {
    final String clusterName;  // required
    final String edsServiceName;  // optional
    final String lrsServerName;  // optional
    final PolicySelection endpointPickingPolicy;  // required
}

For EDS only flow (received from remote resolver):

class XdsConfig {
    final String edsServiceName;  // optional
    final String lrsServerName;  // optional
    final PolicySelection childPolicy;  // required, parser defaults to round_robin
    final PolicySelection fallbackPolicy;  // required, parser defaults to round_robin
}

Since the EDS only flow (XdsLoadBalancer) also delegates load balancing logic to EdsLoadBalancer, it converts XdsConfig to EdsConfig by setting clusterName field to target authority.

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This PR only cleans up LB config(s) (types) for EDS policy, no behavior-wise change.

This PR replaces #6887, which tries to do more cleanup (e.g., refactor FallbackLb, move logic of creating XdsClient object into XdsLoadBalancer) but failed to do so without rewriting the whole set of tests.

[dapengzhang0]

endpointPickingPolicy is not null

[voidzcy]

Done.

[dapengzhang0]

nit: This field is only used in one place, and can be inlined with helper.getAuthority() there. No need to check nullness of helper's authority.

[voidzcy]

Done.