[Filebeat] allow ingest pipelines in YAML format

[adriansr]

This updates the ingest pipeline loading to look at the pipeline definition file extension and use a JSON or YAML decoder depending on the type.

This improves readability a lot, as now it's possible to add comments and multiline strings.

[adriansr]

Didn't add a test, but #11171 includes a YAML pipeline that will be used during tests

[andrewkroh]

This definitely improves readability.

[ruflin]

LGTM, great idea. Looking forward to our first .yml pipeline.

This could even be worth an entry in the dev changelog.

[kvch]

The Filebeat modules generator documentation also needs a follow-up. When generating a new Filebeat fileset, it would be nice to provide a YAML pipeline instead of a JSON. Or maybe making it configurable, if someone prefers JSON over YAML.
@adriansr Could you please create a new issue for the follow-up in Filebeat module generators? Or if you feel like it, you can do it in this PR. :D

[ruflin]

I would keep the default as JSON as otherwise it might be confusing the users. My guess is that most users are not aware that ES API's support YAML.

[kvch]

OK. Still the documentation needs to be updated.