Add NodeAttrsTaildriveShare and NodeAttrsTaildriveAccess to the node capability map, enabling Taildrive file sharing when granted via policy. Add integration test verifying the full cap/drive grant lifecycle.
Updates #2180
Add ConnectToNetwork to the TailscaleClient interface for multi-network test scenarios and implement peer relay ping support. Use these to test that cap/relay grants correctly enable peer-to-peer relay connections between tagged nodes.
Updates #2180