Bugfix for Quickstart Server: Convert RootId to NodeId explicitly with namespace

Proposed changes

The Quickstart Server's NodeHandle class parses a "RootId" from a component's NodeId. The QuickstartNodeManager then tries to use that string as a NodeId. This fails, because the string does not contains a namespace.

This bugfix adds a property "RootNodeId" to the NodeHandle class, which converts the "RootId" to a valid NodeId within the component's namespace. It can be used wherever the root NodeId is needed.

Related Issues

Fixes the exception that occurs when a client subscribes to an "Asset" node.

Types of changes

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[x] Bugfix (non-breaking change which fixes an issue)
[ ] Enhancement (non-breaking change which adds functionality)
[ ] Test enhancement (non-breaking change to increase test coverage)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected, requires version increase of Nuget packages)
[ ] Documentation Update (if none of the other choices apply)

Checklist

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[x] I have read the CONTRIBUTING doc.
[x] I have signed the CLA.
[x] I ran tests locally with my changes, all passed.
[x] I fixed all failing tests in the CI pipelines.
[x] I fixed all introduced issues with CodeQL and LGTM.
[ ] I have added tests that prove my fix is effective or that my feature works and increased code coverage.
[x] I have added necessary documentation (if appropriate).
[x] Any dependent changes have been merged and published in downstream modules.

Further comments

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OPCFoundation / UA-.NETStandard-Samples