enhance `--avail-easyconfig-params` to also accept a (single) easyconfig from which to derive the easyblock

[jfgrimm]

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Currently, -a, --avail-easyconfig-params can either be used:

• on its own, for a list of generic easyconfig parameters
• with -e, --easyblock to also list custom easyconfig parameters specific to a particular easyblock

I think it would be intuitive to either enhance --avail-easyconfig-params, or add another option, which derives the easyblock to use from a specific easyconfig. For example, the output of the following would be identical:

eb -a -e EB_GCC
eb -a GCCcore-12.3.0.eb

As an aside, it might also be nice to be able to request a list of only the custom parameters, with e.g. --avail-easyconfig-params=custom