ARI: Fix a bug where /channels/{channelID}/continue doesn't execute PBX

ARI: Fix a bug where /channels/{channelID}/continue doesn't execute PBX

If /channels/{channelID}/continue is called on a channel that was originated
without a PBX (such as the ARI command POST channel with a stasis application
argument), the channel will not start dialplan execution. This patch will now
run the PBX out of the stasis execution if the channel doesn't currently have
an active PBX upon continuing.

ASTERISK-24043 #close
Reported by: Krandon Bruse
Review: https://reviewboard.asterisk.org/r/3917/
Patches:
    stasis-continue.diff submitted by Krandon Bruse (license 6631)
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Merged revisions 421416 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 421423 from http://svn.asterisk.org/svn/asterisk/branches/13

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@421424 65c4cc65-6c06-0410-ace0-fbb531ad65f3