Feature: #88817 - Make autocomplete selectable in EXT:form backend

See forge#88817

Description

Autocomplete options can already be added to input fields in EXT:form via editing the YAML. This was hard or impossible to do for editors.

The autocomplete tag has to be set on form inputs to be accessibility compliant, where applicable.

The parameter can have arbitrary content according to the HTML standard. However, assistive technology only supports a finite number of values, of which only few are commonly used in contact forms.

Projects that desire additional parameter values can set them via YAML.

Impact

Editors can now select the autocomplete input purpose when editing forms.

In installations that extended the form YAML configuration with keys that are used in this change, the autocomplete field might be overridden and not be displayed in the editor by default.