Add custom description to Credentials

[Widcket]

Changes

This PR adds a custom description to the Credentials class, which is the one displayed when doing print(credentials).

Before (default description)

<A0Credentials: 0x600001290230>

After (custom description)

Credentials(accessToken: "<REDACTED>", tokenType: "Bearer", idToken: "<REDACTED>", refreshToken: nil, expiresIn: 2021-12-05 02:34:23 +0000, scope: Optional("openid profile email"), recoveryCode: nil)

Testing

• This change adds unit test coverage
• This change has been tested on the latest version of the platform/language or why not

Checklist

• I have read the Auth0 general contribution guidelines
• All existing and new tests complete without errors
• All active GitHub checks have passed