Add NDI source/sink/device provider plugin
This plugin is merged from https://github.com/teltek/gst-plugin-ndi with some additional changes on top
- Various bugfixes that were in unmerged pull requests from @vivia and me
- Relicensing from LGPL-2.1 to MPL-2 as agreed with all contributors
- Dynamic loading of the NDI SDK instead of linking to it for easier building and deployment