[azure] Ensure user_agent.original cannot be duplicated

[dmathieu]

What does this PR do?

This is the same fix as #5361 and #6866, but for the user_agent.original field.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.

How to test this PR locally

Run the following query in the dev tools:

POST _ingest/pipeline/logs-azure.signinlogs-1.5.26/_simulate
{
  "docs": [
    {
      "_source": {
        "message": {
          "properties": {
            "user_agent": "NCSA_Mosaic/2.0 (Windows 3.1)"
          }
        },
        "user_agent": {
          "original": "Stone Tablet (Paleolithic)"
        }
      }
    }
  ]
}

Related issues

Screenshots

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• Start Time: 2023-07-10T08:06:19.144+0000

• Duration: 13 min 35 sec

Test stats 🧪

Test	Results
Failed	0
Passed	83
Skipped	0
Total	83

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (10/10)	💚
Files	86.364% (19/22)	👎 -13.636
Classes	86.364% (19/22)	👎 -13.636
Methods	83.333% (155/186)	👎 -15.297
Lines	85.003% (2817/3314)	👎 -8.25
Conditionals	100.0% (0/0)	💚

[elasticmachine]

Package azure - 1.5.26 containing this change is available at https://epr.elastic.co/search?package=azure