feat(assisted-query): Add RPCs necessary for issues search (#102137)

Adds 3 new seer RPC endpoints:
- given an org ID and a list of project IDs, gets all available
attributes
- given an org ID, a list of project IDs, an attribute key, and
optionally a substring to match on, gets all available values for that
attribute
- given an org ID, a list of project IDs, and a query string and other
params, calls and returns the response from the issues list endpoint
(this will have the same data as the frontend gets)

Author: roaga