Add fixed mass 4-momentum behaviors

Describe the potential feature

Add a 4-momentum behavior that doesn't require a temporal component like mass to be present a field on the awkward array and instead is fine with 3 components and a fixed mass.

Motivation

The motivation is that often experiments in root files will not have a mass branch along with pt/eta/phi for example because the mass of a particle is either known or zero. One can say "well just do a full_like operation to add a mass branch" but that A) wastes memory and B) is hacky since the behavior is often attached to vectors created from the root file branches directly like in coffea for example. In coffea we now tell the schemas to create such full-like branches to fill out the mass field.

Possible Implementation

No response

Start by locating the existing 4-momentum behavior and how behaviors are attached to awkward arrays. Compare its requirement for a temporal or mass component with the requested three-component, fixed-mass case. Done means arrays built from pt/eta/phi can use the behavior without a mass branch, while supporting known or zero mass without creating a full-like field.