Allow selectable value in Walker_*Dropdown classes

[jfarthing84]

I think we should be able to select either term_id or term_slug as the output value for Walker_CategoryDropdown class and likewise either ID or post_name for Walker_PageDropdown class.

[jfarthing84]

In researching this, I noticed that many different relating functions use the filter 'list_cats' on many different attributes, which seems misguided, because how do you differentiate between the attributes? Shouldn't each one have it's own filter?

[jfarthing84]

Related #13258

[jfarthing84]

Refresh against [30972]

[boonebgorges]

The wp_dropdown_categories() part of this was duplicated in #30306 and resolved in [31006].

Let's extend the same syntax here, using the 'value_field' parameter name.

[boonebgorges]

In 31338:

Introduce 'value_field' parameter to wp_dropdown_pages().

This parameter allows developers to choose the post field that will be used to
fill in the 'option' attribute of the generated dropdown markup.

See [31006] #30306 for a parallel enhancement in wp_dropdown_categories().

Props jfarthing84.
Fixes #12494.

[DrewAPicture]

In 31355:

Add documentation for the $value_field argument added to wp_dropdown_pages() in [31338].

See #12494.

[DrewAPicture]

In 31356:

Adjust the DocBlock for wp_dropdown_pages() to mark the entire $args parameter array as optional instead of each individual argument.

See #12494.