Add ConfDict operations + Preserve ConfDict order in yaml

[jrapin]

• Added operations ConfDict.ops.DELETE, ConfDict.ops.BEFORE, and ConfDict.ops.AFTER; ConfDict.ops.REPLACE replaces ConfDict.OVERRIDE.
• ConfDict patches apply top to bottom: mapping entries merge content, then move if their anchor exists; unresolved ops remain visible.
• UID generation rejects unresolved ConfDict ops to avoid cache keys from incomplete configs.
• Config YAML dumps preserve key order, while cache uid.yaml checks tolerate key-order-only changes for compatibility.
• Discriminator fields serialize first for more stable/readable config YAML.

[hubertjb]

LGTM, thanks @jrapin! The only thing that I find less clear from the code is the utility of AFTER/BEFORE, not sure you'd want to add a sentence on this in the docstring.