feat(lti13) Username calculation

[florian-jaeger]

Proposed change

Allow username_key value to be a function for username calculation.

Who would use this feature?

Users who want to customise usernames without adding custom LTI claims.

Suggest a solution

Turn username_key into a Union traitlet which can be either Unicode or Callable(function with an ID token parameter, which returns the username in LTI13Authenticator.get_username).

Alternatively a new Callable traitlet could be added instead.

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