Attaching base64 encoded data uris as urls fails validation

Even though base64 encoded data uris are supported in Webex, the [utils validate_uri function](https://github.com/WebexCommunity/WebexPythonSDK/blob/main/src/webexpythonsdk/models/cards/utils.py#L201-L234) for adaptive cards marks them as incorrect.



