Allow QuarkusTest to override base URL of REST client

[snowdrop-bot]

As discussed on Zulip it could be useful if a test could override the base URL of a given REST client.

One use case (as discussed on Zulip) would be for a PACT consumer test, where the test invokes some method on a REST client (directly or indirectly through some service in the application under test), which in turn should direct its request to some PACT mock server, which records the request and returns a canned response.

As a workaround the test currently has to create a REST client manually using RestClientBuilder.

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quarkusio#10126

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$upstream:10126$