Sniff Content-Encoding, don't require Content-Type

[axw]

Motivation/summary

• Don't require the Content-Type header for /intake; assume application/x-ndjson if not specified.
• Sniff Content-Encoding from the request body if the header is not specified

Checklist

• Update CHANGELOG.asciidoc
  - [ ] Update package changelog.yml (only if changes to apmpackage have been made)
  - [ ] Documentation has been updated

How to test these changes

Send a variety of requests to APM Server, checking that they are all accepted:

• Content-Encoding unspecified, uncompressed body
• Content-Encoding unspecified, gzip-compressed body
• Content-Encoding unspecified, deflate-compressed body
• Content-Encoding: gzip, gzip-compressed body
• Content-Encoding: deflate, deflate-compressed body
• Content-Type unspecified
• Content-Type: application/x-ndjson

Check that these requests are rejected:

• Content-Type: invalid
• Content-Encoding: invalid

Related issues

Closes #7271

[mergify]

This pull request does not have a backport label. Could you fix it @axw? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.x is the label to automatically backport to the 7.x branch.
• backport-7./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[apmmachine]

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Build stats

• Start Time: 2022-03-29T08:38:11.376+0000

• Duration: 27 min 0 sec

Test stats 🧪

Test	Results
Failed	0
Passed	3953
Skipped	13
Total	3966

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