Store and use platform audience

[michaelwheeler]

This value can optionally be provided by the platform, in which case it will be used by the tool when requesting an auth token.

[michaelwheeler]

When creating a service token for a client_credentials token request, multiple values from the registration are used:

• The sub property uses the client ID
• The iss property also uses the client ID
• The aud property uses a platform-specified value if one has been provided, otherwise it uses the auth token URL

Construction of the service token in pylti1p3 can be seen here:
https://github.com/dmitry-viskov/pylti1.3/blob/d8fa43e1ace2885e84dfa4067f0f7379aaa86826/pylti1p3/service_connector.py#L54-L69

It seems like it should be enough to add an aud field to the LtiRegistration model and ensure that it is used in the to_registration method when available.