TACTI-003 (Two ACTive Immunotherapeutics): A multicenter, open label, randomized, Phase II trial to investigate a soluble LAG-3 fusion protein, eftilagimod alpha (efti; IMP321) in combination with pembrolizumab (PD-1 antagonist) for first line treatment of subjects with unresectable recurrent or metastatic squamous cell carcinoma of the head and neck (HNSCC)