How to serialize map with dynamic value type

[stefanozanella]

I'm trying to model the class hierarchy to be able to generate a TOML file like the following:

[tableA]
"item1" = "val1"
"item2" = { key = "val2" }

The entries inside the table are dynamic based on user input, so I can't statically add fields to a class representing the table.
Originally I thought of using a Map<String, Any> but Any is not a valid serializable type. An interface implementation would also help I guess but I can't have a string implement a custom interface.

Does anyone have any idea how I could solve this problem?