Add centralPurchasingBody back to eu extension

[ColinMaudry]

Linked to open-contracting/standard#1180.

On hold until open-contracting/standard#1180 is solved, see also open-contracting-extensions/ocds_eu_extension#11 (comment)

[jpmckinney]

This issue only concerns the OCDS 1.2 version of the EU extension. The code still exists in the OCDS 1.1 version.

If we decide in 1180 not to add the role to OCDS, then we probably don't want to add it to any of the OCDS 1.2 extensions that we support either. So, I will defer entirely to 1180.

I suspect that there will be a fair bit of work to do to the profile to upgrade it to OCDS 1.2 (if there is any demand for an upgrade). If there is any work to do around codelists, we can cover it at that time.