[EuiTour] Allow attaching tour steps via DOM selector

[thompsongl]

Two problems and an argument in favor (elastic/kibana#124052):

Right now, it is impossible to link beacons to certain Eui components where the HTML is not accessible from within Kibana - for example, the Columns control in the header of an EuiDataGrid is not currently targetable

Right now, it is difficult to have a tour that spans over multiple elements of the plugin

If EuiTourSteps could instead be linked to elements based on querySelectors or something similar, this would allow for a single tour object to be floated over the entire plugin, which would make the code much easier to follow and would expand the use cases of EuiTour to unimaginable levels! 🔥

This deserves further exploration, but my initial idea is to use EuiWrappingPopover (or a similar API)

[shaunmcgough]

This feature would be great, and would help us with our goal of helping users understand our new Document Explorer in Discover, which is set to be on by default as of 8.2. This will help us get to that goal and we are here to help if needed.