feat: Add quota info to __heartbeat__

[jrconlin]

Closes #947

Description

Adds quota info to the __heartbeat__ result.

Testing

Set quota via env vars like:

    SYNC_ENABLE_QUOTA=true \
    SYNC_ENFORCE_QUOTA=true \
    SYNC_LIMITS__MAX_QUOTA_LIMIT=10000 \
    ... \
    cargo run 

Verify that

> curl http://localhost:8000/__heartbeat__
{"quota":{"enabled":true,"size":10000},"status":"Ok","version":"0.8.3","database":"Ok"}

If quota is not enabled, __heartbeat__ should return something like:

> curl http://localhost:8000/__heartbeat__
{"version":"0.8.2","database":"Ok","quota":{"enabled":false,"size":2097152000},"status":"Ok"}

(The only field that matters here is enabled.)

Issue(s)

Closes #947.