Support the wildcard principal (`"*"`) in STS role config

[Neon-White]

Explain the changes

1. Currently, if a user sets the principal which can assume a role via STS as "*", it always fails since it compares "*" to the email of the requester. This PR checks whether the policy principle is "*" and allows it

Testing Instructions:

1. Create two NooBaa accounts - 'assumed' and 'assumer'
2. Assign a role config to assumed that allows anyone ("principal": ["*"]) to assume it
3. Try to assume it with the credentials of assumer
4. Test the received credentials

• [ ] Doc added/updated
• [ ] Tests added