Implement search database for querying indexed executions and events

https://github.com/deco-cx/durable/blob/main/backends/search.ts

The database for searching executions and events is different from the transactional database. They differ not only in the data model but also in the required capabilities. The transactional database a.k.a WRITE database does not need to support any indexed search, unlike the search database which should support pagination and other complex queries.

The searchDB should be used on the API for querying events and executions