perf: reduce calls to GetWorkspaceByAgentID in GetWorkspaceAgentByID (#21046)

This PR piggy backs on the agent API cached workspace added in an earlier PR to provide a fast path for avoiding `GetWorkspaceByAgentID` calls in dbauthz's `GetWorkspaceAgentByID`. This query is not the most expensive, but has a significant call volume at ~16 million calls per week.

Signed-off-by: Callum Styan <callumstyan@gmail.com>

coderd/agentapi/connectionlog_test.go

@@ -117,6 +117,7 @@ func TestConnectionLog(t *testing.T) {
 				AgentFn: func(context.Context) (database.WorkspaceAgent, error) {
 					return agent, nil
 				},
+				Workspace: &agentapi.CachedWorkspaceFields{},
 			}
 			api.ReportConnection(context.Background(), &agentproto.ReportConnectionRequest{
 				Connection: &agentproto.Connection{