chore: refactor coordination (#15343)

Refactors the way clients of the Tailnet API (clients of the API, which include both workspace "agents" and "clients") interact with the API.  Introduces the idea of abstract "controllers" for each of the RPCs in the API, and implements a Coordination controller by refactoring from `workspacesdk`.

chore re: #14729

agent/agenttest/client.go

@@ -71,7 +71,6 @@ func NewClient(t testing.TB,
 		t:              t,
 		logger:         logger.Named("client"),
 		agentID:        agentID,
-		coordinator:    coordinator,
 		server:         server,
 		fakeAgentAPI:   fakeAAPI,
 		derpMapUpdates: derpMapUpdates,
@@ -82,7 +81,6 @@ type Client struct {
 	t                  testing.TB
 	logger             slog.Logger
 	agentID            uuid.UUID
-	coordinator        tailnet.Coordinator
 	server             *drpcserver.Server
 	fakeAgentAPI       *FakeAgentAPI
 	LastWorkspaceAgent func()