Add data tier validation for Repack workload creation

[germanfgv]

Impact of the new feature
T0 WMAgent

Is your feature request related to a problem? Please describe.
Up until recently, Repack workflows were only able to produce RAW data. Recent changes in #11930 , #1195 and dmwm/T0#4926, allow Tier-0 to configure the data tier that each workflow will produce from Tier-0's configuration file. This opens the possibility of configuration mistakes causing T0 WMAgent to produce the wrong data tiers. As Repack workflows are of vital importance for Tier-0, it makes sense to verify that the data tier is correct.

Describe the solution you'd like
Modify Repack workflow factory to fail if the spec argument for data tier is not in a list of accepted data tiers.

Describe alternatives you've considered
This validation can also be performed in Tier-0 code, as it is there were T0 WMAgent reads the configuration. nmotheless, the actual Repack workflow creation is done using WMCore code. I think it's better to make it impossible to create workflows with the wrong data tier, and not to rely on external checks.

Additional context