Migrate XdsEndpointGroup to client.endpoint (xDS-endpoint pt 2)

[jrhee17]

Motivation:

More classes will be added related to client endpoint, so I propose that we move XdsEndpointGroup from the package com.linecorp.armeria.xds to com.linecorp.armeria.xds.client.endpoint.

Although this PR is mostly refactoring, a new API EndpointGroup#of(ClusterSnapshot) is added since ConfigSourceClient uses GrpcService to decide on the endpoint which will be used.

POC: #5450

Modifications:

• Move XdsEndpointGroup, XdsConstants to com.linecorp.armeria.xds.client.endpoint
• Introduce a new API EndpointGroup#of(ClusterSnapshot)
• Introduce XdsEndpointUtil and move client.endpoint related utility methods from XdsConverterUtil

Result:

• Preparation for adding new features to XdsEndpointGroup is done

[github-actions]

🔍 Build Scan® (commit: 162e835)

Job name	Status	Build Scan®
build-windows-latest-jdk-19	✅	https://ge.armeria.dev/s/755gyscinv4o4
build-self-hosted-unsafe-jdk-8	✅	https://ge.armeria.dev/s/vaxpe5dzfstt2
build-self-hosted-unsafe-jdk-19-snapshot-blockhound	✅	https://ge.armeria.dev/s/bfs5eo4htrhze
build-self-hosted-unsafe-jdk-17-min-java-17-coverage	✅	https://ge.armeria.dev/s/lfxzqbanziamc
build-self-hosted-unsafe-jdk-17-min-java-11	✅	https://ge.armeria.dev/s/k4wy6r6qf3m6u
build-self-hosted-unsafe-jdk-17-leak	✅	https://ge.armeria.dev/s/d73jv3svmq3tq
build-self-hosted-unsafe-jdk-11	✅	https://ge.armeria.dev/s/ijqyjlapkueli
build-macos-12-jdk-19	✅	https://ge.armeria.dev/s/rrdkhg5bfw3m4

[minwoox]

👍 👍 👍

[minwoox]

Let me merge this and #5501 so that @jrhee17 can continue his work in #5503.