Commit Message: Add FULL_SCAN mode to least request load balancer.

By default, the least request load balancer returns the host with the fewest
active requests from a set of N randomly selected hosts.

This introduces a new "full scan" selection method that returns the host with
the fewest number of active requests from all hosts. If multiple hosts are
tied for "least", one of the tied hosts is randomly chosen.

Additional Description:
Risk Level:
Testing:
Docs Changes:
Release Notes:

Added selection_method option to the least request load balancer. If set to FULL_SCAN, Envoy will select the host with the fewest active requests from the entire host set rather than choice_count random choices.

Platform Specific Features:
[Optional Runtime guard:]
[Optional Fixes commit #PR or SHA]
[Optional Deprecated:]
[Optional API Considerations:]

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This PR is a successor to #31146 and other efforts related to added a full scan mode to the least request load balancer.

This approach preserves automatically using full scan mode when choice_count > number_of_hosts. That functionality was previously removed in #30794 because it caused the first host to always be selected if cluster stats were disabled. However, that justification no longer applies because the start index of the full scan is now randomized.

Many thanks to @barroca for initiating this effort and getting it most of the way to the finish line.

Request variables

Key	Value
ref	39b0445d6277a841b080e8ac4e041c866c487e96
sha	825bd5b
pr	31507
base-sha	6ef9425
actor	@jkirschner-hashicorp
message	Add FULL_SCAN selection mode to least request LB...
started	1707237690.447336
target-branch	main
trusted	false

Container image/s (as used in this CI run)

Envoy version (as used in this CI run)