refactors `tailnetAPIConnector` to use the `Dialer` interface in `tailnet`, introduced lower in this stack of PRs. This will let us use the same Tailnet API handling code across different things that connect to the Tailnet API (CLI client, coderd, workspace proxies, and soon: Coder VPN).
chore re: #14729
Closes#14716Closes#14717
Adds a new user-scoped tailnet API endpoint (`api/v2/tailnet`) with a new RPC stream for receiving updates on workspaces owned by a specific user, as defined in #14716.
When a stream is started, the `WorkspaceUpdatesProvider` will begin listening on the user-scoped pubsub events implemented in #14964. When a relevant event type is seen (such as a workspace state transition), the provider will query the DB for all the workspaces (and agents) owned by the user. This gets compared against the result of the previous query to produce a set of workspace updates.
Workspace updates can be requested for any user ID, however only workspaces the authorised user is permitted to `ActionRead` will have their updates streamed.
Opening a tunnel to an agent requires that the user can perform `ActionSSH` against the workspace containing it.
Fixes#12560
When gracefully disconnecting from the coordinator, we would send the Disconnect message and then close the dRPC stream. However, closing the dRPC stream can cause the server not to process the Disconnect message, since we use the stream context in a `select` while sending it to the coordinator.
This is a product bug uncovered by the flake, and probably results in us failing graceful disconnect some minority of the time.
Instead, the `remoteCoordination` (and `inMemoryCoordination` for consistency) should send the Disconnect message and then wait for the coordinator to hang up (on some graceful disconnect timer, in the form of a context).
When an agent receives a node, it responds with an ACK which is relayed
to the client. After the client receives the ACK, it's allowed to begin
pinging.
