funcs/func_curl: Add the ability for CURL to download and store files

This patch adds a write option to the CURL dialplan function, allowing it to
CURL files and store them locally. The value 'written' to the CURL URL
specifies the location on disk to store the file. As an example:

same => n,Set(CURL(http://1.1.1.1/foo.wav)=/tmp/foo.wav)

Would retrieve the file foo.wav from the remote server and store it in the
/tmp directory.

Due to the potentially dangerous nature of this function call, APIs are
forbidden from using the write functionality unless live_dangerously is set
to True in asterisk.conf.

ASTERISK-25652 #close

Change-Id: I44f4ad823d7d20f04ceaad3698c5c7f653c41b0d