Connector builder: Dropdown with available inputs next to each text field

[flash1293]

Context

Split out of #20414

To make it easier to use user inputs in various components of the builder, a dropdown with available inputs should be shown next to each applicable form element. Selecting an entry form the list fills in the placeholder with correct syntax

Acceptance criteria

• Ability to select usable inputs next to each form element
• Selecting an input will append the placeholder to the value of the form element

[flash1293]

Pending for review (just finalized)