add template for gitlab MR

FLEXISIP-597

A priori on ne verra l'effet du template qu'une fois le contenu sur master. Voici le rendu attendu, tout est bien cochable.

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)
  • from the right branch

• docs:

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