Allow visitors to register to an event without having to signup

User story : As a visitor I want to register to a specific event but I don't to create an account as I'm used to not having to do so on other event website I usually go to (eventbrite). We've added the possibility for visitors to answer surveys without having to sign up : https://meta.decidim.org/processes/roadmap/f/122/proposals/14323 Another use case where we think this could be really use full is for registering to an event on the platform. Regarding the specifics of how it would work : - [ ] add a configuration option on the event registration form - [ ] enabling this should make mandatory the creation of a survey to gather the basic necessary data to identify the participant (email, name, consent for receiving emails regarding the event etc.) (as we have to send the unique code) - [ ] We should still incentivize the user sign up at some point. My take is we keep the sign / sign up modal upon clicking on the button join and use it to give the alternative way of continueing without signing up