Implement health check for server.

[davinchia]

What

As part of digging into https://github.com/airbytehq/oncall/issues/192. I realised the OSS Server's health checks aren't actually implemented. This was removed in ##9288. However we do require this to be implemented since we use it in Airbyte Cloud to inform whether the server is connected to the database or not and thus have properly start up.

How

Since this check can potentially be called multiple times a second, it is important for the check to be fast. Because of this, we check the server is working by attempting to list the id column of the workspace table's first row.

Recommended reading order

1. ConfigRepository.java and HealthCheckHandler.java. The rest are just tests.

🚨 User Impact 🚨

Are there any breaking changes? What is the end result perceived by the user? If yes, please merge this PR with the 🚨🚨 emoji so changelog authors can further highlight this if needed.

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[lmossman]

one small typo fix, other than that LGTM