XdsBootstrap leaves a warning log on errors by default

[jrhee17]

Motivation:

While SnapshotWatchers provide a way for users to opt into logging errors, this is not enabled by default.
Additionally, there is no way to notify users that secondary bootstrap resources have not been loaded correctly.
For this reason, I propose that a default SnapshotWatcher be added to a XdsBootstrap where the default watcher logs on errors. This watcher is added to AbstractRoot and bootstrap constructs by default.

In order to ensure parsing failures are propagated, AbstractResourceNode#doOnChanged now also propagates errors to the parent on exception.

Note that transient errors (network errors while fetching resources) are not logged at this level.

Modifications:

• A xDS-dedicated EventExecutor is used for the default event loop.
• XdsBootstrapBuilder is introduced, and users are able to set the default SnapshotWatcher
• The default SnapshotWatcher is added to all bootstrap constructs/AbstractRoot implementations
• If AbstractResourceNode#doOnChanged throws an exception, the error is propagated to the parent node

Result:

• Parsing/configuration errors are propagated to the user-specified SnapshotWatcher
• When an error occurs, a warning log is logged by default

[github-actions]

🔍 Build Scan® (commit: 63d4d4d)

Job name	Status	Build Scan®
build-ubicloud-standard-16-jdk-8	✅	https://ge.armeria.dev/s/ufynfdusuqey6
build-ubicloud-standard-16-jdk-21-snapshot-blockhound	✅	https://ge.armeria.dev/s/l7ec52nho3xv2
build-ubicloud-standard-16-jdk-17-min-java-17-coverage	✅	https://ge.armeria.dev/s/d53isjomrfupq
build-ubicloud-standard-16-jdk-17-min-java-11	✅	https://ge.armeria.dev/s/rpbhpyl2vigqq
build-ubicloud-standard-16-jdk-17-leak	✅	https://ge.armeria.dev/s/uuqrvtoluadg2
build-ubicloud-standard-16-jdk-11	✅	https://ge.armeria.dev/s/a2i5gsa55q5zk
build-macos-latest-jdk-21	❌ (failure)	https://ge.armeria.dev/s/zmtxxch2zrqf4

[ikhoon]

👍👍