Support new returnUrl, returnRisk, and risk parameters

[PeterOrneholm]

Related to #453

Updates the ActiveLogin.Authentication.BankId.Api to support new parameters and return values for the /auth and /sign APIs as per BankID's latest specifications.

• Adds ReturnUrl and ReturnRisk parameters: These optional parameters are now supported in the Request base class, and by extension in AuthRequest and SignRequest. They allow for specifying a return URL and whether a risk indication should be included in the collect response.
• Introduces risk level handling: Adds a Risk property to the Requirement model to set the acceptable risk level for transactions, and to the CollectResponse model to represent the risk level of the order based on the data available.
• Implements BankIdRiskLevels enum: A new file BankIdRiskLevels.cs defines an enum for handling the risk levels (Low, Moderate, High, Unknown) returned in the collect response, facilitating the interpretation of the risk associated with a transaction.
• Updates extension methods: The BankIdAppApiClientExtensions class has been updated to include returnUrl and returnRisk parameters in the AuthAsync and SignAsync methods, ensuring these new options are easily accessible.

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