Consider refactoring the USER_CONFIG_OPTIONS template constant

In the code below the MIP Convert config template is set out, but for CMIP7 we are "breaking" this in several places by modifying it further down stream in MIP Convert .

We might need to pull out the constant into a function that can then modify the template depending on whether we want to work in a "CMIP6" or "CMIP7" style.

https://github.com/MetOffice/CDDS/blob/00d3948e0c83f991ff5141ded546d0c4c0f6079a/cdds/cdds/common/constants.py#L216-L248

Start in cdds/cdds/common/constants.py around lines 216-248 and inspect the downstream MIP Convert code that modifies USER_CONFIG_OPTIONS. Determine how CMIP6 and CMIP7 styles differ and whether the template should be produced per style. Done means the constant's downstream mutation is replaced by a clear style-specific approach without changing the intended configuration output.

Written by the indexing model from the issue text.