There are only a couple endpoints that check peer capabilities. Keeping
permission checks with the code that assumes they were performed, rather
than with the routing layer, feels easier to reason about.
Check that the caller is actually a peer and pass their capabilities via
a context value for handlers that want to check them.
Along with this, simplify the helper handler wrappers that are not
needed for most of the endpoints.
Updates #40851
Signed-off-by: Andrew Lytvynov <awly@tailscale.com>
f15a4f4416