feat(b2bua): add new parameter 'enable-keepalive'

Reviewer guidelines

The following requirements must be met before merging:

• flexisip:

  • tests added
  • related branch pipelines succeeded
  • SHA1 consistency of cherry-picked commits

• linphone-sdk:

  • on a tag (exception for master, which is allowed to use yet-to-be-released versions of the SDK)
  • from the corresponding branch

• docs:

  • CHANGELOG up to date
  • wiki (public documentation) up to date

Closes FLEXISIP-646