Support GIMP's GEGL operations with a custom config argument in new filter API

Environment/Versions

• GIMP version: master
• Package: self
• Operating System: all
• (if Linux) Display system:

Description of the bug

We should probably support the special config files of GIMP's operations. For instance, trying to get the config of "gimp:color-balance" gets us this WARNING:

app/gimp-3.0: Gimp-Plug-In-WARNING: _gimp_param_spec_to_gp_param_def: GParamSpecObject for unsupported type 'GParamObject:GimpColorBalanceConfig'

Maybe we should special-case these by passing through the protocol the args of the config objects instead?

Reproduction

Is the bug reproducible?

Reproduction steps:

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Expected result:

Actual result: