Support different custom serializer for Map values [DATAREDIS-439]

Rob Winch opened DATAREDIS-439 and commented

It would be nice if we could support a custom serializer for specific map values.

For example, in Spring Session it would be nice to allow writing Spring Security's SecurityContext using a custom JSON serialization strategy. However, if the user has not defined custom strategy for their custom object to be able to fall back to JDK serialization. It is likely the API will need to expose additional information in order to determine how to deserialize the values. For example, one might be able to use the key name to determine that JSON or JDK serialization should be used. Alternatively, one might use the value of another key to determine what type of serialization should be used

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