Allow attribution to be converted to Map

[ntsk]

I would like to get the fields of AdjustAttribution in a map like ios_sdk.

https://github.com/adjust/ios_sdk/blob/b1feb897c1d40741ab97b304d90d8817160c57f5/Adjust/ADJAttribution.m#L87-L125

When tracking all the fields, it's hard to check if the fields are added every time this sdk is updated.