Add SafeString As A Valid Message Type

[ardeliatay]

Fix Argument of type SafeString' is not assignable to parameter of type 'string' TS error when passing a SafeString to the flash message service methods:

this.flashMessages.success(htmlSafe('Success!'));
// Argument of type SafeString' is not assignable to parameter of type 'string'

This ticket added SafeString as a valid argument to the flash message methods, but currently the type for message is only a string. This PR introduces a Message type for message arguments to include SafeString to align with that behaviour.

[stukalin]

@ardeliatay well, it's been a couple of years already but do you want to wrap up your pr by any chance?