Return Azure AD auth token in correct format

[jmeunier28]

What does this PR do?

This PR fixes a feature, which was added in #15609 to support Azure Managed Identity authentication for Postgresql Flex/Single Server.

The original implementation used a similar approach to sqlserver/pyodbc to set the token, but that was not necessary and resulted in the incorrect token being passed at connection time. I tested this by updating the code on a local install & was able to verify we are seeing all of the expected integration data for our test postgres instance now:

    postgres (14.2.0)
    -----------------
      Instance ID: postgres:f808f645a7eeae1f [OK]
      Configuration Source: file:/etc/datadog-agent/conf.d/postgres.d/conf.yaml
      Total Runs: 3
      Metric Samples: Last Run: 466, Total: 1,291
      Events: Last Run: 0, Total: 0
      Database Monitoring Activity Samples: Last Run: 1, Total: 3
      Database Monitoring Metadata Samples: Last Run: 1, Total: 1
      Database Monitoring Query Metrics: Last Run: 1, Total: 3
      Database Monitoring Query Samples: Last Run: 21, Total: 77
      Service Checks: Last Run: 1, Total: 3
      Average Execution Time : 116ms
      Last Execution Date : 2023-08-28 19:42:42 UTC (1693251762000)
      Last Successful Execution Date : 2023-08-28 19:42:42 UTC (1693251762000)
      metadata:
        resolved_hostname: dbm-postgres13-flex-server.postgres.database.azure.com
        version.major: 13
        version.minor: 0
        version.patch: 0
        version.raw: 13.0.0
        version.scheme: semver

Motivation

This feature was implemented as part of an effort to support more secure ways to authenticate to databases in the cloud. As we previously added support for IAM, this allows azure users to connect using managed authentication. This is a highly requested customer feature.

[codecov]

Codecov Report

Merging #15701 (197ae22) into master (441957c) will increase coverage by 0.00%.
The diff coverage is 0.00%.

Flag	Coverage Δ
postgres	91.88% <0.00%> (+0.03%)	⬆️

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[github-actions]

Test Results

     12 files       12 suites   15m 0s ⏱️
   228 tests    226 ✔️   2 💤 0 ❌
1 374 runs  1 326 ✔️ 48 💤 0 ❌

Results for commit 197ae22.

[cswatt]

changelog approved by docs