4.x MediaContext single operator support

[romain-grecourt]

• Helidon Version: 4.0.0-ALPHA6

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In 3.x we had this:

WebClient client = WebClient.builder()
        .addReader(JsonpSupport.reader())
        .build();

[tomas-langer]

This seems to be a feature that is not required - it can be easily worked around using a custom media support if needed
(disable discover services, add custom support, or add a support through service loader wiht higher weight than other registered ones).