Add .NET Standard 2.0 Support

[ArachisH]

Features

• Added .NET Standard 2.0 as a target framework, which makes it backwards compatible with various other .NET Framework & .NET Core versions.
  Closes Add support for .NET Framework #26
  https://learn.microsoft.com/en-us/dotnet/standard/net-standard?tabs=net-standard-2-0#select-net-standard-version
• Added a default boolean argument to the INETOptions.Save(string?, string? bool = false) method for stating whether or not to also set the system proxy explicitly.
  Use this option as a last resort when certain applications seem to avoid the running proxy.

Performance Improvements

• Avoid resolving reflection types on every call to generate a certificate, and instead re-use a cache of the required types generated on instantiation.
• Avoid generating a new pair of RSA keys for every certificate, and instead use the same one for every Certifier instance.