Allow host addr to be set from `DAPR_HOST` environment variable

[jeastham1993]

Describe the feature/proposal

Currently when calling the connect function on the dapr Client you must pass in an address. The .NET SDK allows this to be set using a DAPR_HOST environment variable. Add the same functionality in the Rust client.

I'm happy contribute to the SDK to add this feature.

Release Note

RELEASE NOTE: ADD Allow Dapr host to be set from DAPR_HOST environment variable.

[github-actions]

This issue has been automatically marked as stale because it has not had activity in the last 60 days. It will be closed in the next 7 days unless it is tagged (pinned, good first issue, help wanted) or other activity occurs. Thank you for your contributions.