Add login with access token mechanism to PolarionClient

[mbayrak78gh]

Please review :)

[schaveyt]

@mbayrak78gh Rather than using a simple string for the AuthMechanism, please migrate to the following

1. Create an enum AuthenticationMode file and have the enum hold the values of Basic and AccessToken
2. Use the new AuthenticationMode through with it defaulted to Basic and apply the token logic when the enum equals AccessToken