Relative references in $ref are not supported

### Version

4.5.8

### Context

Relative references in $ref are not supported (see OpenAPI spec https://spec.openapis.org/oas/latest.html#relative-references-in-uris) while it worked well with previous OpenAPI library.

The following exception is thrown:

> java.lang.UnsupportedOperationException: Can't resolve 'the relative reference to a OpenAPI file ', only internal refs are supported.
at io.vertx.json.schema.impl.JsonRef.resolveUri(JsonRef.java:344)
at io.vertx.json.schema.impl.JsonRef.resolve(JsonRef.java:211)
at io.vertx.json.schema.impl.SchemaRepositoryImpl.resolve(SchemaRepositoryImpl.java:243)
at io.vertx.openapi.contract.OpenAPIVersion.lambda$resolve$3(OpenAPIVersion.java:86)
at io.vertx.core.impl.ContextImpl.lambda$executeBlocking$1(ContextImpl.java:191)
at io.vertx.core.impl.ContextInternal.dispatch(ContextInternal.java:279)
at io.vertx.core.impl.ContextImpl.lambda$internalExecuteBlocking$2(ContextImpl.java:210)
at io.vertx.core.impl.TaskQueue.run(TaskQueue.java:76)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
at java.base/java.lang.Thread.run(Thread.java:833)

We work with more than one hundred of OpenAPI files. Most of them use relative references to components in other OpenAPI files.
They are provided by a third party.

It would be nice to support relative references from a configuration option.
It would avoid to update OpenAPI files in order to replace relative references by fake absolute ones and to add them by hand as additionalContractFiles in OpenAPIContract.from method.

### Do you have a reproducer?

No

* Link to github project/gist

### Steps to reproduce

### Extra

* Anything that can be relevant such as OS version, JVM version