[Pidgin] #17274: port backend-fs to newer api for farstream relay-info property

[Pidgin]

#17274: port backend-fs to newer api for farstream relay-info property
--------------------+----------------------------------------------
 Reporter:  bellet  |      Owner:
     Type:  defect  |     Status:  new
Milestone:  2.13.0  |  Component:  libpurple
  Version:  2.12.0  |   Keywords:  backend-fs2 farstream relay-info
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 Current farstream requires relay-info to be passed to farstream as a
 GPtrArray instead of a GValueArray. In current code, the turn server
 defined in the gui is silently ignored, with just this notice in the
 debugging log:

 {{{
 (08:37:01) GLib-GObject: unable to set property 'relay-info' of type
 'GPtrArray' from value of type 'GValueArray'
 }}}

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Ticket URL: <https://developer.pidgin.im/ticket/17274>
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