Support override of powershell module feeds

[benbp]

Adding an override list of ps modules we want to redirect to our devops mirror feed.

Also simplified the module existence check (still thread safe), but I'm thinking that maybe the double existence check was there so we wouldn't have a giant lock request backlog if a ton of threads all do a module check at the same time (given it takes 1 sec to do the check if it's behind the lock). @mikeharder any recollection on this? From https://github.com/Azure/azure-sdk-tools/pull/6521

[azure-sdk]

The following pipelines have been queued for testing:
java - template
java - template - tests
js - template
net - template
net - template - tests
python - template
python - template - tests
You can sign off on the approval gate to test the release stage of each pipeline.
See eng/common workflow

[benbp]

@scbedd @weshaggard latest update adds fallback support, so we can query multiple package feeds until one succeeds.

[azure-sdk]

The following pipelines have been queued for testing:
java - template
java - template - tests
js - template
net - template
net - template - tests
python - template
python - template - tests
You can sign off on the approval gate to test the release stage of each pipeline.
See eng/common workflow

[benbp]

@scbedd @weshaggard added another commit to unregister the repository after install, it's causing some weird overlap issues with dotnet tool install later in the pipelines, possibly b/c the url format has to be slightly different. Still trying to figure out what's actually going on here.

Installing test-proxy version 1.0.0-dev.20241101.1 Unhandled exception: NuGet.Protocol.Core.Types.FatalProtocolException: Failed to fetch results from V2 feed at '[https://pkgs.dev.azure.com/azure-sdk/public/_packaging/azure-sdk-tools/nuget/v2/FindPackagesById()?id='azure.sdk.tools.testproxy'&semVerLevel=2.0.0'](https://pkgs.dev.azure.com/azure-sdk/public/_packaging/azure-sdk-tools/nuget/v2/FindPackagesById()?id=%27azure.sdk.tools.testproxy%27&semVerLevel=2.0.0%27) with following message : Response status code does not indicate success: 401 (Unauthorized - No local versions of package 'azure.sdk.tools.testproxy'; please provide authentication to access versions from upstream that have not yet been saved to your feed. (DevOps Activity ID: D6280451-EFE3-48AA-BF7C-7A31FBB5EF7E)). ---> System.Net.Http.HttpRequestException: Response status code does not indicate success: 401 (Unauthorized - No local versions of package 'azure.sdk.tools.testproxy'; please provide authentication to access versions from upstream that have not yet been saved to your feed. (DevOps Activity ID: D6280451-EFE3-48AA-BF7C-7A31FBB5EF7E)). at System.Net.Http.HttpResponseMessage.EnsureSuccessStatusCode()

[azure-sdk]

The following pipelines have been queued for testing:
java - template
java - template - tests
js - template
net - template
net - template - tests
python - template
python - template - tests
You can sign off on the approval gate to test the release stage of each pipeline.
See eng/common workflow