Update UDS example with socket paths determined at runtime

## Feature Request

### Motivation

Workaround and Rust education for closure capture requirement that currently only a static path is used:

https://github.com/hyperium/tonic/blob/c30cb783b9e5eaa4ea05003d9cacac733d135c58/examples/src/uds/client.rs#L16-L28

### Proposal

Use the `ustr` crate to remove reliance on only a static hard-coded path.