Moves the logic that handles the registration for serialization of OpenAPI schema class to smallrye-openapi-extension

[machi1990]

After merging of 1389, @kenfinnigan came with a better idea of moving the said logic from resteasy-extension into smallrye-openapi-extension. This PR incorporates the proposed changes.

cc @gsmet.

[gsmet]

There is a formatting issue: run mvn formatter:format in the openapi extension directory: that will fix it.

[machi1990]

There is a formatting issue: run mvn formatter:format in the openapi extension directory: that will fix it.

Thanks, I'll be running the command onwards. My IDE is still giving me some headache with the formatting ide-config.

[gsmet]

The easy way to get everything in order is to run mvn clean install before pushing any PR. It runs the test suite but it also reformat everything.

[machi1990]

The easy way to get everything in order is to run mvn clean install before pushing any PR. It runs the test suite but it also reformat everything.

Golden rule :-)

[gsmet]

Merging, thanks!