Dial: Add function to append already-created channel.

The Dial API takes responsiblity for creating an outbound channel when
calling ast_dial_append(). This commit adds a new function,
ast_dial_append_channel(), which allows us to create the channel outside
the Dial API and then to append the channel to the ast_dial structure.

This is useful for situations where the channel's creation and dialing
are distinct operations. Upcoming ARI early bridge work will illustrate
its usage.

ASTERISK-25889

Change-Id: Id8179f64f8f99132f80dead8d5db2030fd2c0509