Cherry-pick #23334 to 7.x: [Filebeat] Add postgresql csv fileset

[jsoriano]

Cherry-pick of PR #23334 to 7.x branch. Original message:

As a configuration option, create another pipeline inside the module to
parse CSV log files.

Although it's necessary to configure the database to emit such logs,
there is some advantages: configured properly, some events like
statement timeout and lock timeout will display the query in the
same event, opposed to multiple lines, one with the error message and
other with statement.

What does this PR do?

This PR creates a new fileset/pipeline inside PostgreSQL module. This new fileset parses and create events based on CSV log files from postgresql.

Why is it important?

PostgreSQL logs weren't designed to be parsed, so, without very complicated logic, it's not possible to encapsulate all information in just one event. One example of that is statement timeout:

2021-01-04 02:07:15.470 UTC [13470] [user=postgres,db=postgres,app=[unknown]] ERROR:  canceling statement due to statement timeout
2021-01-04 02:07:15.470 UTC [13470] [user=postgres,db=postgres,app=[unknown]] STATEMENT:  SELECT pg_sleep(100)

Using the CSV log, those 2 lines become 1:

2021-01-04 02:07:15.470 UTC,"postgres","postgres",13470,"127.21.1.7:28182",5ff27834.349e,1,"SELECT",2021-01-04 02:06:44 UTC,16/4181895,0,ERROR,57014,"canceling statement due to statement timeout",,,,,,"SELECT pg_sleep(100)",,,""

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Author's Checklist

• Documentation

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• Build Cause: Pull request #24088 opened

• Start Time: 2021-02-17T15:05:00.759+0000

• Duration: 51 min 45 sec

• Commit: 04f7c5b

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