indexer management client, health check, basic requests

[hopeyen]

Description

• Add options to provide indexer management server endpoint (INDEXER_MANAGEMENT_SERVER_ENDPOINT) and coverage level for automatically upgradeable subgraphs (AUTO_UPGRADE)
• basic health check to the endpoint at radio initialization
• requests: get all indexing rules, post offchain sync request
• added None to coverage level
• Unit tests relies on a running indexer server so temporarily commented out until there is a mock server

No changes required for the users.

Issue link (if applicable)

Resolves #23

Checklist

• Are tests up-to-date with the new changes?
• Are docs up-to-date with the new changes? (Open PR on docs repo if necessary)