Support Security Requirements

[wildarch]

Add field "security" to the Spec struct to support Security Requirements

[wildarch]

I believe the type can be simply BTreeMap<String, Vec>. I'll verify this and send a pull request with the changes.

[yovoslav]

This one is a bit old and the merged changes from #17 don't seem to be in the latest changes in master, I have opened #57 to add Security Requirements to V3