Make SearchFilter.PackageType singular rather than plural

[zivkan]

Bug

Fixes: NuGet/Home#14941
Fixes: NuGet/Home#14936
also follow up of #7395

Description

We had a community contribution recently: #7395
In the PR, I made two comments about things that I think should be improved, so I implemented it myself, in addition to some additional changes:

• BREAKING CHANGE Remove the old IEnumerable<string> PackageTypes property, and add a new string PackageType property to SearchFilter. See the linked issue for more information.
• Add a SupportsPackageTypeFiltering property to PackageSearchResource, so callers can check whether the feed supports package type filtering before calling SearchAsync, given SearchAsync will throw if PackageType is not null or empty on feeds that don't support it.
• Remove a log message added in the community PR. While the log might be interesting while developing and testing the package type filtering feature specifically, I believe the message would not provide value in the general sense (when customers are using NuGet normally).
• Several minor perf improvements (reducing unnecessary allocations).

PR Checklist

• Meaningful title, helpful description and a linked NuGet/Home issue
• Added tests Almost entirely a refactoring, not adding new functionality or fixing bugs.
• Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc. N/A