Fix server security array unmarshaling error

[lmendes86]

Fixes #294

The AsyncAPI v3.0 specification defines servers[].security as an array, but the Server struct incorrectly used *SecurityScheme instead of []*SecurityScheme, causing JSON unmarshaling errors when parsing specs with server security definitions.

Changes:

• Updated Server.Security field to []*SecurityScheme
• Updated generateMetadata() to iterate over security array
• Updated setDependencies() to iterate over security array
• Added test case in test/v3/issues/294/

[lerenn]

Sorry about the delay, I had some changes in my life this 2 past months, I'll take a look ASAP.