Stops header: add initial train velocity and service cadence fields

[Pivouane]

Description and goal

In the new time stops table header, we should be able to set the initial velocity of train, and to set the service cadence, using trailing content input components.

See also the mockups in Sketch ↗.

Acceptance criteria

• The train initial velocity can be edited, with a km/h unit, named "Initial velocity" in English, "Vitesse initiale" in French
• The service cadence can be edited, with a min unit, named "Service cadence" in English, "Cadence de la mission" in French

[Pivouane]

Part of this done in #17201