Simplify + Allow custom MDC lookup

[SylvainJuge]

Allow sub-classes of JUL EcsFormatter to provide their own MDC lookup (which then allows to set it in the APM agent).
Remove the JUL MDC implementation as it is not required after all.

[apmmachine]

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• Start Time: 2022-08-08T14:25:12.283+0000

• Duration: 7 min 31 sec

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Test	Results
Failed	0
Passed	244
Skipped	0
Total	244

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