Add controller-led worker auth flow (#2413)

This adds an action to the API that allows pulling out an activation
token that can be given to a worker in its config (directly or via env
or file). The worker can pass this along with its normal request in
place of its normal nonce and the server will automatically accept it.

Co-authored-by: Irena Rindos <irenarindos@users.noreply.github.com>

internal/proto/controller/api/resources/workers/v1/worker.proto

@@ -72,6 +72,10 @@ message Worker {
   // https://github.com/hashicorp/nodeenrollment
   google.protobuf.StringValue worker_generated_auth_token = 150 [json_name = "worker_generated_auth_token"]; // @gotags: `class:"public"`
 
+  // Output only. An activation token that can be given to a worker to correlate
+  // it to the created resource.
+  google.protobuf.StringValue controller_generated_activation_token = 151 [json_name = "controller_generated_activation_token"]; // @gotags: `class:"secret"`
+
   // Output only. The number of connections that this worker is currently handling.
   google.protobuf.UInt32Value active_connection_count = 160 [json_name = "active_connection_count"]; // @gotags: `class:"public"`