Add support for CryptographyContext factories

[dusk0r]

Currently there is no way to control the creation of (S/MIME / OpenPGP) CryptographyContexts. I propose optional factory methods.

[jstedfast]

Why not just subclass the context you want and register it's Type?

[jstedfast]

I much prefer the way it is now...

[dusk0r]

Because you can't use non-static external resources.

[jstedfast]

Can you give me an example of what you are trying to accomplish that can't be done by subclassing?

[jstedfast]

Keep in mind that your PR breaks existing users of MimeKit... :(

[jstedfast]

Actually, n/m, I looked at your PR again and it doesn't...

[dusk0r]

We have several ASP.NET webapps. On startup they fetch a private S/MIME Key to sign outgoing mails. I don't want to embed or store the certificate on the machine.

Right now my best solution is to fetch the key and store it into a static property. Then subclass the context and load the key from that property. But I don't like that solution.

[jstedfast]

Since it doesn't break API/ABI, I'm willing to merge this, but can you fix the code formatting/indenting to match the rest of the code? Thanks.

[jstedfast]

Awesome, thank you.