RESTEasy Reactive client: add User-Agent

[FroMage]

Describe the bug

Most websites straight-up refuse REST calls without a User-Agent. GitHub's API does. We should probably set one by default, otherwise users have to do:

@ClientHeaderParam(name = "User-Agent", value = "MP REST") 
@RegisterRestClient
@Path("/")
public interface GithubClient {
}

And the problem is that if this value is missing, all calls will fail with an exception which does not log the GitHub response, so it takes quite a while to figure out why it fails (see #22213).

Expected behavior

No response

Actual behavior

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How to Reproduce?

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Output of uname -a or ver

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Output of java -version

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GraalVM version (if different from Java)

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Quarkus version or git rev

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Build tool (ie. output of mvnw --version or gradlew --version)

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Additional information

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[quarkus-bot]

/cc @geoand, @stuartwdouglas

[geoand]

How about also adding the ability to do this in config?

[sberyozkin]

By the way @FroMage, this will also work with the oidc-token-propagation-reactive (filter) once it is available, will open a sep issue

[geoand]

#22236 is what I have in mind

[FroMage]

Hang on, this doesn't appear to add a user-agent… Ah, it's #22228, fine!