Add ECS console appender for spring boot

[ipalbeniz]

This PR adds a console appender for spring boot (#137)

[eyalkoren]

Thanks!!
Please just confirm that you tested that this exact file works as expected when imported.

[apmmachine]

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• Start Time: 2021-07-20T07:35:37.023+0000

• Duration: 7 min 33 sec

• Commit: 0d9674c

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Total	171

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[eyalkoren]

You can also add it to the xml example in https://github.com/elastic/ecs-logging-java/blob/master/docs/tab-widgets/ecs-encoder.asciidoc

[eyalkoren]

run elasticsearch-ci/docs

[eyalkoren]

@ipalbeniz once you confirm that you manually tested that this works as expected when taken as is from the repo, I can go ahead and merge.

[ipalbeniz]

@eyalkoren I just tested it and works as expected.