TypeScript: Suggest introducing a type for metadata.keyspaces

I'd like to suggest introducing a type for the keyspace metadata. This way it would be possible for consumers to extend the type.

Currently:

Propose:

Then consumers could do something like:

Without that new type, TS will complain about about subsequent property defs having the same type.

Also, `strategyOptions` seems like a good candidate to have on packaged types.