Tenant (customer) identifier that conveys the tenant scope for the request
(DoiT API Design Standards §15).

Resolution when the header is absent:

• A key scoped to exactly one tenant resolves to that tenant automatically; the header is
  optional.
• A principal that can access more than one tenant (e.g. DoiT-employee keys) must supply the
  header. Without it the request is rejected with 400 and code tenant_id_required — the
  server does not guess across tenant scopes.

When the header is present but conflicts with the tenant the key is scoped to, the request is
rejected with 400 and code tenant_id_mismatch.