Make ENSRainbow to return it's config even before it is ready

[tk-o]

The ENSRainbow config data model includes a field for the total count of rainbow records.

We could make ENSRainbow config available even before the ENSRainbow instance become ready by:

1. Removing the count of rainbow records from the ENSRainbow config data model.
2. Updating ENSRainbow so that it could return its public config as soon as it starts up (just from environment variables) without any need to load or process data beyond that.
3. Updating packages/ensnode-sdk/src/stack-info/ensnode-stack-info.ts which is currently under development in Merge ENSApi /api/config endpoint into /api/indexing-status endpoint #1671 so that the ensRainbow field stops being optional since then it could be loaded super fast, even in a cold-start scenario.

[coderabbitai]

🔗 Related PRs

#1843 - Onchain event handler preconditions [merged]

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[djstrong]

ENSRainbow serve does not use env variables, it only accepts path to database and read label set info from it.
We can enforce setting the variables and when db is ready validating if they match.