Implement fallback MDC implementation for JUL

[SylvainJuge]

Part of https://github.com/elastic/apm-agent-java/issues/2492

Adds a fallback MDC implementation for JUL that can be used by the APM agent, usage within ECS logging will only read any key/values in the MDC, values will be populated by the APM agent.

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Checklist

• [x] adding at least some comments to explain that the agent might rely on the code structure (the agent should test for that too)
• [x] how to properly size the custom MDC size and load factor ?

[apmmachine]

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#### Build stats * Start Time: 2022-08-01T15:26:59.793+0000 * Duration: 7 min 33 sec #### Test stats :test_tube: | Test | Results | | ------------ | :-----------------------------: | | Failed | 0 | | Passed | 245 | | Skipped | 0 | | Total | 245 |

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[felixbarny]

@eyalkoren correct me if I'm wrong but I think the idea was to not rely on the MDC from slf4j at all anymore. Instead remove MdcSupplier and add a MDC class with static methods similar to org.slf4j.MDC.

[eyalkoren]

@eyalkoren correct me if I'm wrong but I think the idea was to not rely on the MDC from slf4j at all anymore. Instead remove MdcSupplier and add a MDC class with static methods similar to org.slf4j.MDC.

Correct, that's the preferable option described in https://github.com/elastic/apm-agent-java/issues/2492#issue-1158621899, and the less preferable alternative described is to rely on slf4j MDC