Add structured representation for form fields in AIPageContent mojom

form_name
form_field_type
field_name (string
field_value (string)
select_options:
  select_option_value (string)
  select_option_text (string)
  select_option_selected
placeholder (string)
is_checked
is_required

Also expose GetPlaceholderValue() to TextControlElement, which is required to populate the `placeholder` field in
AIPageContentFormFieldData.

Fixed: 381879263
Change-Id: I9b9812bef6045109be43aa50f38d4fdde411b48f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6194296
Commit-Queue: Abigail Klein <abigailbklein@google.com>
Reviewed-by: Dominic Farolino <dom@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1413602}