Implement support for custom verify callbacks

This patch was authored by @Lukasa and released by @0xTim.

Motivation

When using NIOSSL it is sometimes necessary to completely take over the
certificate verification logic. NIOSSL exposes a callback for this, but
it's currently hidden from Vapor users. We should let them get access to
this callback.

Modifications

• Added the callback to the HTTPServer configuration struct.
• Plumbed the callback through.
• Added some invalid test certs to the resources for the tests.
• Added a test to confirm the override functions correctly.

Result

Users can override client cert validation.