Authorizes an existing connection again. The connection keeps its id.

For a Pipedream connection, creates a Pipedream Connect session scoped to the
existing connection's external user and returns the data needed for the
frontend to call connectAccount with an accountId. The frontend then calls
the reconnect confirm endpoint.

For a native OAuth connection, returns the provider's authorization URL. The
OAuth callback completes the reconnect: it stores the new tokens on the same
connection, clears its disconnect_reason and redirects to the same success
page as a new connection.