Adding brownfield scenario to add session hosts to existing host pool

[danycontre]

Overview/Summary

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This PR fixes/adds/changes/removes

1. New brownfield scenario to add new session hosts.

Breaking Changes

1. none

   Testing Evidence

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As part of this Pull Request I have

• [ x] Read the Contribution Guide and ensured this PR is compliant with the guide
• [ x] Ensured the resource API versions in .bicep file/s I am adding/editing are using the latest API version possible
• [ x] Checked for duplicate Pull Requests
• [ x] Associated it with relevant GitHub Issues
• [ x] (AVD LZA Team Only) Associated it with relevant ADO Items
• [ x] Ensured my code/branch is up-to-date with the latest changes in the main branch
• [ x] Performed testing and provided evidence.
• [ x] Updated relevant and associated documentation (e.g. Contribution Guide, Module READMEs, Docs etc.)