Add repository parameter to diffusion.findsymbols method

Summary: Ref T7977. Add a repository parameter to the diffusion.findsymbols Conduit method to allow restricting search results to the specified repository.

Test Plan: Queried the Conduit endpoint.

Reviewers: avivey, epriestley, #blessed_reviewers

Reviewed By: avivey, epriestley, #blessed_reviewers

Subscribers: eadler, avivey, Korvin, epriestley

Maniphest Tasks: T7977

Differential Revision: https://secure.phabricator.com/D12663