Allow additional token data to be added to message

I am curious if anyone has tried to tackle a token handler that might work like this:

$message->addContext('salutation', 'Greetings ' . $name);

And then in the message template could be a token like: [message:contexts:salutation]

Automated tests could likely be added to tests/src/Functional/MessageTemplateUiTest.php

@amitaibu

I changed the field context name.

Tests added in tests/src/Kernel/MessageTest.php