Virtual methods for CognitoUser

[WajahatAliAbid]

*Issue #, if available:
aws/aws-aspnet-cognito-identity-provider#151
Description of changes:
Mock testing CognitoUser using Moq fails because it needs virtual methods to create a proxy with [1].

Currently the workaround is to write a subclass to wrap around the CognitoUser class as follows

public class CustomCognitoUser : CognitoUser
    {
        public CustomCognitoUser(string userID, string clientID, CognitoUserPool pool, IAmazonCognitoIdentityProvider provider, string? clientSecret = null, string? status = null, string? username = null, Dictionary<string, string>? attributes = null) : base(userID, clientID, pool, provider, clientSecret, status, username, attributes)
        {
        }
        public virtual new Task<AuthFlowResponse> StartWithAdminNoSrpAuthAsync(InitiateAdminNoSrpAuthRequest adminAuthRequest)
        {
            return base.StartWithAdminNoSrpAuthAsync(adminAuthRequest);
        }
    }

This PR makes the methods of CognitoUser virtual so we can write mock test cases for CognitoUser class.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[normj]

This PR is merged as part of version 2.1.0 that went out today.