Per namespace security

[alex-zywicki]

As a continuation of #110 It would be very helpful to be able to customize security schemes on a per namespace level.
Currently asyncapi does not support this, so it would need to be in the form of a x-security specification.

One of the main questions I see is how per namespace security would interact with global security. Would it override global level security? or would both have to pass?

[dedoussis]

Thanks for the contribution @alex-zywicki! This functionality has been released as part of version 0.6.0. Closing the issue.