Add support for .NET Framework to BrotliCompressionProvider (when available)

[tpeczek]

Thanks to @khellang contributing #217 and #216 we now have BrotliCompressionProvider working for .NET Core 2.1.

This issue is to add support for .NET Framework when it arrives (dotnet/corefx#29760)

[muratg]

We won't be doing this.

[Sojaner]

We won't be doing this.

And why is that?

[Tratcher]

Because the next release won't support .NET Framework. aspnet/Announcements#324

[Sojaner]

Because the next release won't support .NET Framework. aspnet/Announcements#324

Thanks for the clarification.