This change introduces a new private protocol, MDCTextControlTextField, which inherits from MDCTextControl. This protocol adds one property, of a new enum, MDCTextControlTextFieldSideViewVerticalPosition. This type helps MDCBaseTextFieldLayout determine how to vertically position side views. The purpose is to allow MDCUnderlinedTextField to vertically align its side views with the text rect, as opposed to vertically centering them in the container, like the filled and outlined styles do. I'm not sure about the name `MDCTextControlTextFieldSideViewVerticalPosition`, but other ones I considered that were maybe more clear were too ridiculously long.
PiperOrigin-RevId: 316028095
