## Description 

Adds a `checkpoint_timestamp_ms` to the watermark task and uses it in a
new health check endpoint function. The health endpoint checks for two
things
- if there is a DB connection, otherwise it returns code 500
- if the last known checkpoint timestamp is within an acceptable buffer.
It subtracts the current timestamp from the checkpoint timestamp, and
checks if the value is larger than the provided query param
`max_checkpoint_lag_ms` or a default value, and it returns code 504,
GATEWAY TIMEOUT in that case.

How to query this endpoint:
`curl -X GET "http://127.0.0.1:8000/health" -i `
Set the check for max checkpoint time lag to 10s. If it returns 503,
then the checkpoint is behind.
`curl -X GET "http://127.0.0.1:8000/health?max_checkpoint_lag_ms=10000"
-i`

## Test plan 

Added a new test.

`cargo nextest run --features pg_integration -- test_health_check`

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## Release notes

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- [ ] Protocol: 
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