UsageIdentifierKeys added into AuthResponse for use in Built-In Authorizers

[bunnyhugdev]

I added in a usageIdentifierKey attribute into the AuthResponse class and modified the to_dict method to serialize it if it exists. This allows my built-in authorizer to inject API Key values for tracking in Usage Plans. My use case involves translating existing header values into API Keys, instead of forcing my clients to provide an additional header and key.

Issue #, if available:

Description of changes: app.py AuthResponse class updated to add the attribute in the contstructor and serialize in the to_dict method. tests/test_app.py updated to include test case where usageIdentifierKey attribute is used. docs/api.rst updated to reflect the new attribute.

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