feat(inputs.opcua+inputs.opcua_listener) Support node id string in config

[LarsStegman]

Use Case

Being able to use the node id string in the config files will make it much easier for more novice users to configure Telegraf.

Expected behavior

Users have to write

{name="ProductUri", namespace="0", identifier_type="i", identifier="2262"}

Actual behavior

They should be able to write

{name="ProductUri", node_id="ns=0;i=2262"}

Additional info

No response

[powersj]

This sounds like a nice improvement!