Ability to set the default Zyte API Parameters for every Request

[BurnzZ]

Currently, the parameters are passed on a per-request basis.

Users should be able to:

1. enable Zyte API for all request
2. set the default parameters for every Zyte API request

[BurnzZ]

The PR in #13 only partially addresses the points (2) of the issue as it doesn't introduce the mechanism of how to properly enable every scrapy.Request to go through Zyte API without having the need to set the zyte_api param in Request.meta.

There are also things we need to figure out like in the #10 (comment) thread.