Setup infra for spark-observer

[bajtos]

As part of #97 and #106, we need to build a new service to listen for Meridian events and update spark-stats database. The current design, where spark-evaluate manages and updates the spark-stats database is no longer serving us well.

Going forward, we want to implement the following design:

• A single repository to hold spark-stats database schema migration scripts, the code writing the stats (smart-contract event observer), the code reading the stats (REST API server)
• A monorepo-like structure that makes it easy to install dependencies for all components, run tests for all components, etc. One command to perform the action for all components.

Tasks

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1. Expose as node module filecoin-station/spark-impact-evaluator#12
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2. Migrate spark-stats database to spark-evaluate database. Update spark-evaluate and spark-stats database connection string in Fly secrets to use the new database.
3. Implement a new monorepo structure in the spark-api repository.
4. Rework the spark-stats repository to use this new monorepo structure too.
5. Add a new component spark-observer to the spark-stats repository, setup Fly.io deployment
6. Add database migration scripts to spark-stats repository.
7. Modify spark-stats to read from two databases: spark-evaluate for existing data, spark-stats for the future data created by spark-observer

Nice-to-have improvements

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1. Add npm scripts like test:api and lint:publish to spark-api
2. Add npm scripts like test:stats and lint:observer to spark-stats

[juliangruber]

@bajtos I'm assigning you as DRI, so this issue has one. Feel free to change it if you disagree