interop: Add initial indexing api spec

[ajsutton]

Description

Adds a spec for the first method in the "indexing backend" RPC API. This is designed to specify a standard API for tools that indexing logs across chains to optimise lookups for sequencing and verifying nodes. Written in a way that tries to be independent of any particular design for the indexer.

Still todo:

• GetCrossUnsafe method
• NextDeriveTask method
• OnDerived method
• CrossSafe method
• TryFinalize method

[tynes]

Thoughts on calling this the "Superchain API"?

[ajsutton]

I'm slightly against that because I think this is an entirely optional thing that could be used as part of implementing interop. Calling it Superchain API makes it sound like a critical component. While our implementation will depend on it, it's certainly possible to implement interop without it or with a different API if you make different architectural decisions.

[ajsutton]

@protolambda I think we moved away from this approach again and should just close this right?

[protolambda]

@ajsutton I am not entirely sure yet. We'll likely run into something like this as part of devnet 2 reorg support planning, although maybe we can simplify it. I'm fine with keeping this open for now.