[FR] Per-unit teamcolor

Spot the hero unit:

Having special units denoted by a custom teamcolor is a classic convention. Not just heroes either, for example a game could denote buffs or elemental affinity or whatever this way.

The request is to either expose `Spring.SetUnitTeamColor(unitID, r, g, b, a)` or, if this is already possible via shaders, to add a widget with such shader (and a similar WG/GG API) to basecontent for easy inclusion.

Start by checking the existing unit rendering and shader support relevant to custom team colors, then review whether a Spring.SetUnitTeamColor(unitID, r, g, b, a) API or a basecontent widget is the better route. If shader-based, also determine the required WG/GG API surface. Done means a usable per-unit color customization path for heroes, buffs, or similar effects.

Written by the indexing model from the issue text.