.Net: Add AssemblyAI file service Remake from #5964

[RogerBarreto]

Motivation and Context

Description

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[RogerBarreto]

@Swimburger, added minor changes to overall code.

• Stopped using JsonDocument in favor of JsonElement for performance
• Changed structure from Files to Services, all public services should be in the root namespace.
• Changes Client to be Core in line with our other connectors.
• Update Namespaces on UnitTests.
• Sealing specialized class.

[Swimburger]

Those changes look great. I didn't know about the perf difference, thanks for sharing and updating!