Add native line item support

[leandro-otani]

Description

It would be great to have native line item setup functionality.

Based on the official prebid docs, the line-item part mainly differs in the custom creative format, so I assume steps done by the tool would be:

1. Request* a custom creative format ID (e.g. YAML input).

2. Define the NativeStyle, as in GAM example;

3. Instantiate a new creative from format/template ID

4. Move with line-item setup, specifying this new format in expected creatives & attaching the new creative to the line-item.

*Note that GAM API docs doesn't seem to have any methods for creating it, so I guess it's either using one of the standard IDs (10004400 or 10004520) along with the mandatory macros, or requesting the ID from the user; It's also worth mentioning that, if there is already a NativeStyle in place for said ID, it would be necessary to include labels to specify where it would be delivered.

What do you guys think?

If you guys have any/questions considerations, please let me know!