[6.x] Augment appended form config fields in emails

[lwekuiper]

#13111 augmented appended form config fields for Antlers, but only in Forms\Tags. The email view builds form_config the same way and was missed, so {{ form_config:* }} in an email template still receives raw, unaugmented values.

For scalar fieldtypes this goes unnoticed, since the raw and augmented values render the same. It shows up as soon as the appended field is something else: a Bard field renders its raw array (Array to string conversion), and an assets field hands you the filename string instead of an asset, so {{ form_config:download:permalink }} comes out empty.

This applies the same one-line change as #13111 to Forms\Email::addData(), plus a test mirroring the one added there.

Found while adding a per-form download to the submitter's email, where the appended config field points at the asset to link.