Fix bug in Third Party REST API ingest pipeline

[leehinman]

What does this PR do?

Fixes bug in Third Party REST API ingest pipelines where a rename of
host.name would fail because it was already set. Also moves third
party api processing to a separate pipeline.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
  - [ ] If I'm introducing a new feature, I have modified the Kibana version constraint in my package's manifest.yml file to point to the latest Elastic stack release (e.g. ^7.13.0).

How to test this PR locally

need to install Splunk & ingest data into that, then configure these
integrations. Bug isn't visible with pipeline tests.

Related issues

• Closes Apache integration with Third Party REST API not working #1146

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[elasticmachine]

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Build stats

• Build Cause: Pull request #1201 updated

• Start Time: 2021-06-28T15:11:45.324+0000

• Duration: 67 min 19 sec

• Commit: b05f9c4

Test stats 🧪

Test	Results
Failed	0
Passed	521
Skipped	0
Total	521

Trends 🧪

[P1llus]

LGTM, thanks a lot for this work, it cleans up some of the technical debt from my earlier changes.
Initially this was planned once a package could share pipelines, but I think it was good to do it already now!

[P1llus]

Just a general question, I am quite sure that set always correctly overwrites fields right? Since event.original is already set at this point?

[marc-gr]

Correct, set overwrites any previous value