cubicweb #2800850 add rpm spec file [done]

We need to automate building rpms, shipping a spec file with the cw source is a step towards that.

prioritynormal
typeenhancement
done in3.16.2
load1.000
load left0.000
closed by#280c32415059 [packaging] add spec file to build RPMs (closes #2800850)
patch[packaging] add spec file to build RPMs [applied]