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Since the Qt5 generator did not support any security features, I added basic API key authentication. It supports multiple keys as well as key in http header or key as URL query.

EDIT: Added basic authentication with username and password as well.
EDIT2: Updated Readme and added basic bearer token authentication.

Sample headers for verification:

With key in HTTP header:

--> POST /pet HTTP/1.1
--> Host: localhost:3000
--> User-Agent: OpenAPI-Generator/1.0.0/cpp-qt5
--> api_key: special-key
--> Content-Type: application/json
--> Accept-Encoding: identity
--> Content-Length: 75
--> Connection: Keep-Alive
--> Accept-Language: en-US,*

With key in URL query:

--> POST /v2/pet/1606131759633?api_key=special-key HTTP/1.1
--> Host: localhost:3000
--> User-Agent: OpenAPI-Generator/1.0.0/cpp-qt5
--> Content-Type: application/x-www-form-urlencoded
--> Accept-Encoding: identity
--> Content-Length: 20
--> Connection: Keep-Alive
--> Accept-Language: en-US,*

With basic auth (base64 encoding of "demo:p@55w0rd"):

--> POST /v2/pet/1606141504633 HTTP/1.1
--> Host: localhost:3000
--> User-Agent: OpenAPI-Generator/1.0.0/cpp-qt5
--> Authorization: Basic ZGVtbzpwQDU1dzByZA==
--> Content-Type: application/x-www-form-urlencoded
--> Accept-Encoding: identity
--> Content-Length: 20
--> Connection: Keep-Alive
--> Accept-Language: en-US,*

With bearer token ("special-bearer-key"):

--> POST /v2/pet HTTP/1.1
--> Host: localhost:3000
--> User-Agent: OpenAPI-Generator/1.0.0/cpp-qt5
--> Authorization: Bearer special-bearer-key
--> Content-Type: application/json
--> Accept-Encoding: identity
--> Content-Length: 4
--> Connection: Keep-Alive
--> Accept-Language: en-US,*

ping @wing328