feat: changed MailFinderService to load all TemplateTypes at once

[tinect]

⚠️BREAKING CHANGE for MailFinderServiceInterface ⚠️

• possible successor of Make it compatible with MailTemplateEntity #85
• Used Struct to add possibility to extend it - is it worth it? no
• the files were read and parsed through the loader also before

[tinect]

@shyim @Schrank something like this I was thinking of.

Preventing the user from being able to edit the template could be really hard, while we support shopware 6.5 - 6.7 in one version - and you know the challenge regarding fields in admin xD

[tinect]

@Schrank would this work for you?

[Schrank]

My colleague tested it and it works as expected <3 Feel free to merge it!

[Schrank]

And thank you very much for the work <3

[tinect]

@shyim ? :-)

[schneider-felix]

Well no you'll also need to add the use-Statement for Entity :D