[DigitalTwins] Update user-agent value

The User-Agent header needs to be consistent with the guidelines:
https://azure.github.io/azure-sdk/general_azurecore.html#telemetry-policy

This means the UA value should be:
`azsdk-python-digitaltwins-core/1.1.0 <platform_info>`
To do this, the `sdk_moniker` parameter needs to be passed to the generated client:
```python
sdk_moniker = "digitaltwins-core/{}".format(VERSION)
```