E3-1668: Open62541 build

This creates libopen62541 as an e3 module. The build is done using cmake (I tried otherwise, but this seemed the simplest) during the prebuild stage as before, but then it is packaged separately as an e3-module.

I have tested that opcua builds correctly against this version, but I have not tested the functionality yet.

Update: I have built this locally and then run opcua's tests against it. All tests passed.