Support more extra data/parameters

[jor123]

Currently extra_info can be used to add extra parameters. But only SystemInfo is documented as available.

I started creating a CustomDimension class based on the impl of SystemInfo, but I also needed other parameters (session control/override).
I ended up just passing a list of dicts into the extra_info parameter, which works but is kinda 'undocumented API'?
I think the request method should either allow a list 'extra data class instances' (Requestables)? Or document the 'list of dicts' as acceptable input...?

[patrys]

For what it's worth I would rather ditch the SystemInfo part and let you pass a simple dict as extra_info. I don't think it's worth wrapping each and every possible parameter in a separate class.