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:

• [x] GetCrossUnsafe method
• [x] NextDeriveTask method
• [x] OnDerived method
• [x] CrossSafe method
• [x] TryFinalize method