Add a `fields` key to `wp_get_sites()`

[flixos90]

I think it would be useful to have a new fields key in the $args parameter for wp_get_sites() to allow returning specific fields only, basically to allow developers to create site lists in the way they need them more quickly. The idea is based on a similar concept to what we can currently do in post or term queries as well.

Possible use-cases that I have in mind would be:

• ids (returns an array of site IDs)
• count (returns the site count as integer; docs would need to be adjusted here since it's no array)
• id=>domain (returns array where key is site ID and value is its domain)
• id=>path (similar to id=>domain, but with path)
• id=>network_id (similar to id=>domain, but with network ID)

Maybe there's more, but in my opinion that would be a good starting point, covering many possible usages.

[flixos90]

patch with a possible implementation

[flixos90]

35697.diff​ is a take on that enhancement.

[ocean90]

Related: #35791

[jeremyfelt]

Now that get_sites() and WP_Site_Query are available, let's close this out.

[jeremyfelt]

This feature would still be possible in get_sites() / WP_Site_Query, so reopening.

[flixos90]

I don't think this is really necessary to have (although I initially opened it). Closing for now.