features: Add transfer initiation options.

Adds additional control options over the transfer
feature functionality to give users more control
in how the transfer feature sounds and works.

First, the "transfer" sound that plays when a transfer is
initiated can now be customized by the user in
features.conf, just as with the other transfer sounds.

Secondly, the user can now specify the transfer extension
in advance by using the TRANSFER_EXTEN variable. If
a valid extension is contained in this variable, the call
will automatically be transferred to this destination.
Otherwise, it will fall back to collecting the extension
from the user as is always done now.

ASTERISK-29899 #close

Change-Id: Ibff309caa459a2b958706f2ed0ca393b1ef502e3