[keyserver] Update updateRelationshipsResponder to handle RelationshipRequest

Summary:
Make necessary changes to updateRelationshipsResponder and updateRelationships to support RelationshipRequest which is union of previous TraditionalRelationshipRequest and FarcasterRelationshipRequest.

• Using request.action instead of destructuring makes it clear to flow in request.action === relationshipActions.FARCASTER_MUTUAL branch that request.userIDsToFID is available
• We get userIDs using userIDsToFID if farcaster request and pass to new Set(requestUserIDs) which is the only place request.userIDs is "directly" consumed

Depends on D11610

Test Plan: Send both traditional and farcaster requests, set breakpoint in updateRelationships, ensure that values are as expected at each step.

Reviewers: ginsu, tomek, varun, will

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11611