Added custom hostname support to the OPCUA Discovery Server

[XOlifreX]

In the project I work on currently we have a LDS deployed on a Kubernetes cluster in the cloud. When trying to register a server on it, every time the LDS would take its local hostname, a name that cannot be resolved by any server who's on a different network.

This pull request adds a way to define custom hostnames to the OPCUADiscoveryServer class in the same manner the OPCUAServer can. Now you can pass a custom hostname that does get resolved outside of the network.

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