.Net: feat: Added support for .yml extensions in C# SDK

[WretchedDade]

Updated CreatePluginFromPromptDirectoryYaml to support files with both .yaml AND .yml extensions.

Motivation and Context

This fixes #11322 by adding support for the .yml file extensions when loading YAML files from a directory in the C# SDK.

Description

Updated the CreatePluginFromPromptDirectoryYaml method to load files from the plugin directory twice. Once with the *.yaml search pattern as before, and once with the *.yml search pattern.

Updated the tests to include a file with the .yml extension to ensure that the method works as expected and compatibility isn't lost in the future.

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 😄

[WretchedDade]

Looks like I forgot to push the change of removing the blank line to appease the formatter. I have pushed that change now.

[WretchedDade]

For some reason the format action still failed, for .Net 8 specifically. I ran dotnet format locally and it didn't change anything 🤷🏻. The complaints came from the lines where I was spreading the arrays in the collection initializer, so I converted that to simply .Concat(...) them. Hopefully that will appease the formatter. Can you reapprove @markwallace-microsoft?