Allow MFs (module-function pairs) in rabbit_mnesia_tables_to_khepri_db attr

[LoisSotoLopez]

Proposed Changes

As discussed in #15559 it would be useful to allow a module-function pair as value for the rabbit_mnesia_tables_to_khepri_db module attr.

This PR implements so.

Types of Changes

• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)
• Build system and/or CI

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