Deserialize and serialize unknown fields

[marandaneto]

Source #426 (review)

the idea is that unknown fields in the protocol are always kept and not lost across ser/deser.

Example:

Dart SDK deserializes an envelope that contains a new field called newField.
Right now this field is ignored as its not mapped in the SentryEvent class.

The idea is to keep all these unknown fields in a data bag with the key name and key value.

When the Dart SDK serializes again the very same event, it reads this data bag and writes all those values back to the JSON with the original property name and value.

[marandaneto]

similar to getsentry/sentry-java#1497