Token usage restrictions per field no longer possible, maybe make TokenContext::definitions() extensible

[michb]

In the Notification Center before 2.0, it was possible to define a token and restrict its use to certain fields. Is this still possible in any form?
To my understanding of TokenDefinitions and TokenContext I could not find out if such a thing is still possible.

To make it clearer, here is a quick example:
We have our own notification type and provide a list of emails separated by comma. Now we need to define them with EmailTokenDefinition.
But to prevent the editor from leaking the emails, we only allow this token in the bcc field, and in no other field. That is currently not possible.

I think if the TokenContext::definitions() was extensible, we could achieve everything we need. Or this bundle could add a BccEmailTokenDefinition and a TokenContext::BccEmail, but that would only solve this one particular case.

Another case is that we have implemented an SMS gateway. To make this user-friendly, we would like to have a PhoneNumberTokenDefinition, but it is currently not possible to map this to a TokenContext that can be used in the tl_nc_language dca definition.

[Toflar]

You can adjust the context that is allowed per field using nc_context. So your fields need to have a different context (it's just a string) - for example phone-number and then you can use the GetTokenDefinitionClassesForContextEvent to define, which token definition classes are responsible for that context. So there again, you can create your own PhoneNumberTokenDefintion for example.