Separated core projects from sample projects in solution level

[aehomay]

made the solution structure a bit more clear by separating source code for core and sample projects. This will help people that are getting the first touch with the OPC UA to not confuse core source from sample source and also remind them that core could be replaced by consuming NuGet

[CLAassistant]

All committers have signed the CLA.

[AlinMoldovean]

@aehomay ,

Could you sign the license agreement ?
It is needed to accept your contribution.

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ AlinMoldovean
❌ aehomay
You have signed the CLA already but the status is still pending? Let us recheck it.