Add a generator exported function for events.

[maximelb]

Provide an exported function to generate native (maps) Events through a channel instead of generating the whole file all at once.

This allows for more efficient processing of very large evtx file to stream JSON out.

The generator takes a file descriptor in to read the evtx, and returns a channel of event+error, a close function (to abort processing) and an error for the initial reading of the evtx file.

[CLAassistant]

All committers have signed the CLA.

[scudette]

We are still parsing entire chunks at the time so it really doesn't make much difference for very large evtx files right?

Here is the corresponding code in Velociraptor https://github.com/Velocidex/velociraptor/blob/d62b72887b3b2364554cdba9645570b43c91f347/vql/parsers/event_logs/evtx.go#L103

I guess this is more of a convenience function but it wont make much difference to memory performance during parsing of large evtx files.