pbx: Add support for autohints.

This change introduces the concept of autohints. These are hints
which are created as a result of device state changes occurring within
the core. When this happens a hint will be created (if it does not
exist already) using the device name as the extension.

For example if a device state change is received for "PJSIP/bob"
and autohints are enabled on a context then a hint will exist in
that context for "bob" with a device of "PJSIP/bob".

For virtual or custom device states the name after the type will
be used. For example if the device state of "Custom:bob" changes
then a hint will exist in that context for "bob" with a device of
"Custom:bob".

This functionality can be enabled in extensions.conf by placing
"autohints=yes" in a context.

ASTERISK-25881 #close

Change-Id: I7e444c7da41b7b7d33374420fec658beeb18584e