search API - add context metadata

[umbrellaassociates]

Search API currently only returns results and pages properties.

there is no place to add additional contextual data to the results.

One use case is the so called "Zookie" in SpiceDB or a general time-based token that represents a state of the resultset.
In SpiceDB, the Zookie/token is used by the application and used for later PDP/PIP requests, making some advantage of the databases caching mechanism.

Another possible use case for context-data could be application specific statistics, like the number of records.

[ogazitt]

I could see adding an optional "context" field in the response (a peer to "results" and "page"), which stores additional contextual information returned by the PDP. I don't think we need to standardize what goes in this (at least for v1).

[mtrimpe]

@davidjbrossard This is included in the search pagination PR but that hasn't been merged yet.