NDI Discovery Server Registration

[mattg66]

Description

It should be possible to provide NDI Discovery Servers to CasparCG so that the NDI output functionality can work in larger environments that span multiple subnets. The SDK already includes this capability.
https://docs.ndi.video/all/getting-started/white-paper/discovery-and-registration/discovery-server

Solution suggestion

Add configuration entries that allow multiple discovery servers to be specified.

[mattg66]

Opened #1702 to address this.