[Elasticsearch] Add Pipeline configuration

[aleksmaus]

What type of PR is this?

/kind feature

Any specific area of the project related to this PR?

/area outputs

What this PR does / why we need it:

Adds configurable ingest pipeline for Elasticsearch output. For more documentation refer to: https://www.elastic.co/guide/en/elasticsearch/reference/current/ingest.html

This is super useful when users need to specify the ingest pipeline for the data transformation.

Special notes for your reviewer:
Added one FIXME: comment https://github.com/falcosecurity/falcosidekick/pull/981/files#diff-63512941b06c3ecd46ca56b6745027a3d30b9ff012c5e1e9d1508ee4e22df39fR102.
There is a potential race condition when c.EndpointURL gets set in the output request sending code, since this is a member variable that can be accessed from multiple go routines. I fixed this for Elasticsearch output, and noticed another place in Sumologic output https://github.com/falcosecurity/falcosidekick/blob/master/outputs/sumologic.go#L24
I can look at addressing that FIXME if I have time later to avoid the similar mistakes in other outputs.

[Issif]

You're right for the race condition, like we discussed before, the client implementation is pretty naive, but easy to extend, even by beginner. Beside that, all is good to me. Thanks again.

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aleksmaus, Issif

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[poiana]

LGTM label has been added.

Git tree hash: c87a9f3aa6ddc840bf68faef1dc74259ccfa50b6