[ISSUE-5705] Support for degraded health

[seonWKim]

Motivation:

Add support for degraded health.

Modifications:

• Add HealthStatus enum which can represent the health status of a server
• Add healthStatus() method in HealthChecker for fine-grained representation of server's status
• Add degradedResponse which shows that the server's status is being degraded.

I think adding new health check service which returns HealthStatus(instead of boolean) might be an alternate option. But because this PR is before any reviews, I've just implemented feature on top of HealthCheckService.

Result:

Users can now receive notification for degraded server status like below

{"healthy":true, "degraded":true}

• Closes https://github.com/line/armeria/issues/5705. (If this resolves the issue.)
• Users can now identify degraded server status in addtion to healthy and unhealthy.

[github-actions[bot]]

🔍 Build Scan® (commit: 03b58998b0f516851ae4ecbe5bbed4d0e936ef0e)

Job name	Status	Build Scan®
build-windows-latest-jdk-21	✅	https://ge.armeria.dev/s/fkx54r7t6ept2
build-self-hosted-unsafe-jdk-8	✅	https://ge.armeria.dev/s/qklmc4f7wlxho
build-self-hosted-unsafe-jdk-21-snapshot-blockhound	✅	https://ge.armeria.dev/s/lygfz7up3orp2
build-self-hosted-unsafe-jdk-17-min-java-17-coverage	✅	https://ge.armeria.dev/s/gkzmsnwkinmiw
build-self-hosted-unsafe-jdk-17-min-java-11	✅	https://ge.armeria.dev/s/zuhkqmmkrmhmy
build-self-hosted-unsafe-jdk-17-leak	✅	https://ge.armeria.dev/s/ozlftpkhomptq
build-self-hosted-unsafe-jdk-11	✅	https://ge.armeria.dev/s/mkzctgrd4muym
build-macos-12-jdk-21	✅	https://ge.armeria.dev/s/obrj3klg3uk3k

[seonWKim]

Removed the [ISSUE-5741] prefix in the commit messages.

[seonWKim]

Question) what do you think of just combining the two HealthCheckUpdateHandler and adding similar logic as HealthCheckService?

Looks better. I don't think we need to split them into different classes. 👍