encoding/gob: add RegisteredTypes()

[laskoviymishka]

Starting from Go 1.23, direct access to encoding/gob.nameToConcreteType is no longer possible due to the removal of internal symbol linking. While this change improves encapsulation, it also removes the ability to retrieve a list of all registered types in the gob package.

Relates: #67401

Closes: #71602

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