Several WebProxy-related fixes (#50368)

Several WebProxy-related fixes (#50368)

- Split browser from non-browser to avoid CA1416 suppressions, UnsupportedOSPlatform attributes, and PNSEs when used from browser
- Cache domain name and local addresses for use in IsLocal, along with clearing of the caches upon network change detection
- Removing redundant IPAddress.IsLoopback check that's covered by previous Uri.IsLoopback check
- Change IsMatchInBypassList to use string interpolation, such that the non-default port case will get better implicitly with C# 10 interpolated string improvements