[IMP] survey: allow to link a user answer to its user input email field

PURPOSE

Allow some questions to be linked to customer data. We would like to have an
email field filled with user info / updating user info automatically in the survey.

SPECIFICATION

For now, we only want to be able to collect the user email address for a specific
question of the survey. For that purpose, add a ``save_as_email`` field on the
question model.

If this option is checked, the answer to that specific question will override the
"email" stored on the survey.user_input allowing live udpate of user input
based on given information. If multiple questions are marked as such, the
last (not empty) answer will prevail.

If user input already has an email (invitation-based surveys for example)
prefill the answer with the email so that people don't have to retype it.

GOING FURTHER (aka, incoming)

With live mode: maybe some additional fields will need to be saved
nickname
avatar

LINKS

Task 1919290
PR #41429

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>
Co-authored-by: Aurélien Warnon <awa@odoo.com>
Co-authored-by: Thibault Delavallée <tde@odoo.com>