Create a gateway framework

[Jannis]

We want something similar to the indexer-service framework introduced in graphprotocol/indexer-rs#80.

This will require extracting a number of modules from the graph-gateway source code into its own crate. The list below tracks the completion of this:

• Provider (indexer) discovery (generalize all subgraph-specific bits)
• Gateway block cache (for making queries deterministic)
• Reporting via Kafka
• Prometheus metrics
• Indexer selection (generalize all subgraph-specific bits)
• Indexer status tracking
• Budgeting & indexer cost tracking
• GeoIP support

This list may not be complete.

[LNSD]

This comment is related to PR #808:

How do you envision a JSON-RPC gateway using a "request selector"?

It's clear to me in the subgraph gateway. One client can pick one subgraph either:

• By subgraph ID: The gateway decides the deployment version depending on the indexers' performance info.
• By deployment ID: The subgraph version is pinned, and the gateway makes decisions based on indexers' performance info.

Could you explain what needs to be multiplexed and selected in a JSON-RPC gateway?