bluetooth: ots: Add object name write capability

Add the ability to perform a write on the object name GATT
Characteristic with a notification callback to the application
that the name has been written.

In order for this operation to work the memory backing the
object name must be modifiable. To prevent forcing the user
to always allocate 120 bytes for the name, the maximum name
length is changed from a define to a configuration parameter.

Signed-off-by: Abe Kohandel <abe.kohandel@gmail.com>