Minimal content negotiation

[bart-vmware]

Description

This PR re-adds content negotiation in a minimal (hard-coded) way. Every actuator uses the same, single media type, which is not extensible. An error is returned when an incompatible Content-Type or Accept header is used.

My concern is that adding content negotiation support after the release of Steeltoe v4 will be a breaking change. This PR ensures that incompatible Content-Type and Accept headers are blocked. This enables Steeltoe to support additional content types in the future and introduce interfaces and/or configuration for third-party actuators to participate in the process, without suddenly starting to fail on requests that used to succeed.

I haven't done any testing with Cloud Foundry, Prometheus or Spring Boot Admin yet. We should verify they are compatible with the changes in this PR before merging it.

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