feat(b2bua): Add Priority headers for MWI NOTIFY messages in the B2BUA.

Adding this Priority headers allow the flexisip proxy to choose whether or not to send push notifications for MWI NOTIFY messages according to the number of urgent messages notified in the content of the MWI.

Depends on linphone-sdk!5641 (merged)

FLEXISIP-953

Reviewer guidelines

The following requirements must be met before merging:

flexisip	done	N.A.	Tests added
	done	N.A.	Related MR pipelines succeeded
	done	N.A.	SHA1 consistency of cherry-picked commits
linphone-sdk	done	N.A.	On a tag... (Except for master, which is allowed to use yet-to-be-released versions of the SDK)
	done	N.A.	...From the corresponding branch
docs	done	N.A.	CHANGELOG up to date
	done	N.A.	wiki (public documentation) up to date
	done	N.A.	internal specifications up to date