Adds optional `DiscoveryUrl` knob to OpcUaClientDriverOptions. When set,
the driver runs `DiscoveryClient.CreateAsync` + `FindServersAsync` +
`GetEndpointsAsync` against that URL during InitializeAsync and prepends
the discovered endpoint URLs (filtered to matching SecurityPolicy +
SecurityMode) to the failover candidate list. De-duplicates URLs that
appear in both discovered and static lists (case-insensitive). Discovery
failures are non-fatal — falls back to statically configured candidates.
The doc comment notes that FindServers requires SecurityMode=None on the
discovery channel per OPC UA spec, even when the data channel uses Sign
or SignAndEncrypt.
Closes#278
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
