Expose KeyUsage in X509Certificate

[bcgit]

Expose KeyUsage in X509Certificate

[jstedfast]

FWIW, I ended up implementing my own extension for X509Certificate to get this functionality in MimeKit.

A set of bit flags:

https://github.com/jstedfast/MimeKit/blob/master/MimeKit/Cryptography/X509KeyUsageFlags.cs

and the extension method:

https://github.com/jstedfast/MimeKit/blob/master/MimeKit/Cryptography/X509CertificateExtensions.cs#L215

[Genbox]

Actually ended up not using X509Certificate as most of the API is not really functional in my case.
I'd rather see the KeyUsage bits exposed as a flags enum (like @jstedfast did) and store the enum value, instead of checking against an array of bool.

I would have made a pull request for this as I did implement the change in the BC library, however, I was not keen on changing 30 unit tests as well.