feat(inputs.opcua): Add namespace URI support

[skartikey]

Summary

Adds support for OPC UA namespace URIs (nsu=) as an alternative to namespace indices (ns=) in both inputs.opcua and inputs.opcua_listener plugins.
This enhancement allows users to configure nodes using stable namespace URIs instead of numeric indices that can change when OPC UA servers restart, making configurations more robust and portable.

New Configuration Option

• Added namespace_uri field to node and group configurations
• Supports both regular nodes and event nodes
• Works alongside existing namespace (index) field with mutual exclusion validation

Example Usage

Before (using namespace index):

 [[inputs.opcua.nodes]]
   name = "Temperature"
   namespace = "3"
   identifier_type = "s"
   identifier = "Temperature"

After (using namespace URI):

 [[inputs.opcua.nodes]]
   name = "Temperature"
   namespace_uri = "http://opcfoundation.org/UA/"
   identifier_type = "i"
   identifier = "2255"

Breaking Changes
None - this is purely additive. Existing configurations using namespace continue to work unchanged.

Checklist

• No AI generated code was used in this PR
• AI generated code used in this PR follows the InfluxData Policy on AI-Generated Code Contributions

Related issues

resolves #14417

[telegraf-tiger]

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[srebhan]

Just a question for understanding the matter. As we touch this part anyway, wouldn't it be easier do define something like

[inputs.opcua_listener.metric]
  name = "my_metric"
  [inputs.opcua_listener.metric.fields]
    ServerStatus = "ns=0;i=2256"
    MyInfo  = "nsu=http://opcfoundation.org/UA/;s=foo"
  [inputs.opcua_listener.metric.tags]
    location = "main building"
    device = "typesetter"

This is more compact, less error prone (you cannot define duplicate fields) and easier to graps as it defines how a metric looks like and uses the notation used for Node IDs in OPCUA, isn't it?

[srebhan]

Ok, let's implement the feature in the current form and add the more compact configuration later if we want to...

[srebhan]

Thanks @skartikey!