fix: AutomationAccount - Credential et al

[AlexanderSehr]

Description

• Updated credential module to align with specs (singular resource deployment per module)
• Added export for all types
• Optimized defaults for child module

Depends on #3397

Pipeline Reference

Pipeline

Type of Change

• Update to CI Environment or utilities (Non-module affecting changes)
• Azure Verified Module updates:
  • Bugfix containing backwards-compatible bug fixes, and I have NOT bumped the MAJOR or MINOR version in version.json:
    • Someone has opened a bug report issue, and I have included "Closes #{bug_report_issue_number}" in the PR description.
    • The bug was found by the module author, and no one has opened an issue to report it yet.
  • Feature update backwards compatible feature updates, and I have bumped the MINOR version in version.json.
  • Breaking changes and I have bumped the MAJOR version in version.json.
  • Update to documentation

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[microsoft-github-policy-service]

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