Récupération des données de renvoi d'appel via le FAM

Allow all flexiapi connection to use the same http2 client, created from the global::flexiapi configuration.

If configured, make FlexiAPI calls to retrieve call diversion information for every call made. For now, FlexiAPI information is cached for a small amount of time (30s) to avoid too many unnecessary calls while maintaining a sense of real-time configuration changes.

FLEXISIP-912

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