Implement locality/priority based load balancing

[jrhee17]

Motivation:

This pull request attempts to implement the default EdfLoadBalancerBase which is the default load balancer used in envoy. Most load balancer implementations in envoy derive from this base class.

The basic load balancer supports locality weighted load balancing and priority levels.

Note that not all functionality (namely zone aware load balancing, etc..) have been implemented for simplicity.
Additionally, SubsetLoadBalancer has not been implemented in this PR for easier reviewing. The previous SubsetLoadBalancer will be replaced in the next (and final) pull request.

This pull request mostly focuses on updating the LoadBalancer state when a ClusterSnapshot is updated.
The flow is as follows:

1. ClusterEntry#accept is called, which indicates a ClusterSnapshot has been updated
2. A PriorityStateManager computes endpoints per priority and locality.

• https://github.com/envoyproxy/envoy/blob/bdff856f461c2261305b8303285e83d6f2cec627/source/common/upstream/upstream_impl.cc#L2075

3. A PrioritySet is created. This PrioritySet contains a map of Integer -> HostSet where a HostSet contains host information for each health/locality.

• https://github.com/envoyproxy/envoy/blob/bdff856f461c2261305b8303285e83d6f2cec627/source/common/upstream/upstream_impl.cc#L2099
• https://github.com/envoyproxy/envoy/blob/bdff856f461c2261305b8303285e83d6f2cec627/source/common/upstream/upstream_impl.cc#L610

4. A DefaultLbStateFactory is created which creates a LbState for the DefaultLoadBalancer. A LbState is a convenience object used to avoid potential race conditions. The LbState is replaced atomically from the perspective of DefaultLoadBalancer.

• In terms of envoy source, each load balancer registers a callback which listens for HostSet updates. Once a HostSet is updated, the load balancer state is updated.
• https://github.com/envoyproxy/envoy/blob/bdff856f461c2261305b8303285e83d6f2cec627/source/common/upstream/load_balancer_impl.cc#L149-L154
• Essentially, a range from 1~100 is assigned to each priority where healthy endpoints are prioritized.

5. Lastly, the load balancer chooses a HostSource, and subsequently a host from the selected HostSet

• https://github.com/envoyproxy/envoy/blob/bdff856f461c2261305b8303285e83d6f2cec627/source/common/upstream/load_balancer_impl.cc#L1173
• https://github.com/envoyproxy/envoy/blob/bdff856f461c2261305b8303285e83d6f2cec627/source/common/upstream/load_balancer_impl.cc#L891-L892

ref: #5450

Modifications:

• ClusterEntry now creates a PrioritySet for each ClusterSnapshot update
  • In order to create a PrioritySet, utility classes PriorityStateManager, PriorityState, HostSet, UpdateHostsParam have been created.
  • Utility classes EndpointUtil, EndpointGroupUtil have been created
• DefaultLoadBalancer has been introduced. The functionality of this LoadBalancer is equivalent to EdfLoadBalancer.
• DefaultLbStateFactory has been introduced to facilitate creating a state for DefaultLoadBalancer. The LbState created by DefaultLbStateFactory is intended to be updated atomically from the perspective of DefaultLoadBalancer.

Result:

• Priority and locality based load balancing is now available for XdsEndpointGroup

[github-actions]

🔍 Build Scan® (commit: 27d2ed4)

Job name	Status	Build Scan®
build-windows-latest-jdk-21	✅	https://ge.armeria.dev/s/zl7ltuc45uwta
build-self-hosted-unsafe-jdk-8	✅	https://ge.armeria.dev/s/ioi6sowwadzhg
build-self-hosted-unsafe-jdk-21-snapshot-blockhound	✅	https://ge.armeria.dev/s/i4cppmwqedc62
build-self-hosted-unsafe-jdk-17-min-java-17-coverage	✅	https://ge.armeria.dev/s/eg4djqce4emjq
build-self-hosted-unsafe-jdk-17-min-java-11	✅	https://ge.armeria.dev/s/njo4h6aub7fws
build-self-hosted-unsafe-jdk-17-leak	✅	https://ge.armeria.dev/s/nkwll6xlbqlzq
build-self-hosted-unsafe-jdk-11	✅	https://ge.armeria.dev/s/wfvyb3xhm5j2k
build-macos-12-jdk-21	✅	https://ge.armeria.dev/s/w6vrv7vo4he6i