tailcfg/peercap, types/appctype: prepare for conn25 grants

This change contains the protocol changes needed to support
describing authorization for conn25 apps. As a temporary transition
measure during development, app configurations can disable authorization
enforcement.

Updates tailscale/corp#40076

Change-Id: I3183c10374aacb0048f6632c384f71f758c20f2f
Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>