mon: MonCommands.h: have 'auth' read-only operations require 'x' cap

This reintroduces the same semantics that were in place in dumpling prior
to the refactoring of the cap/command matching code.

We haven't added this requirement to auth read-write operations as that
would have the potential to break a lot of well-configured keyrings once
the users upgraded, without any significant gain -- we assume that if
they have set 'rw' caps on a given entity, they are indeed expecting said
entity to be sort-of-privileged entities with regard to monitor access.

Fixes: #7919

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>