Versions for pixi-build-backends so that we have to break less.

[tdejager]

Let's start versioning the build backends, because the breakage is already becoming annoying. Agter the dicussions we've come to the following conclusions.

• For the cmake and python backends let's just start versioning against breakage
• For the rattler-build backend we are going to couple the name against the recipe version so conda-recipe and when there is a v2 we'll make a seperate backend. That way we can use major/minor.