[Attestation] remove hardcode endpoint in extension cloud

- If the issue is to do with Azure CLI 2.0 in-particular, create an issue here at [Azure/azure-cli](https://github.com/Azure/azure-cli/issues)

### Extension name (the extension in question)
Today there's hardcode endpoint `https://attest.azure.net` in [extension code](https://github.com/Azure/azure-cli-extensions/blob/master/src/attestation/azext_attestation/generated/_client_factory.py#L28), pls remove it from extension, but add it in cli core repo _cloud.py. Then extension read endpoint from context.

### Description of issue (in as much detail as possible)

-----