filter_parser: Add parameter to nest parsed fields under

[RaJiska]

Adds a Nest_Under parameter to the parser filter so that parsed fields can be nested into a field instead of being unpacked in at the roof level of the map.

Addresses #8551, and #7775.

Example configuration file:

[SERVICE]
    Parsers_File /fluent-bit/etc/parsers.conf

[INPUT]
    Name  Dummy
    Dummy {"log":"error: my error","element":"{\"a\":\"b\",\"c\":{\"x\":\"y\"}}"}
    Tag   dummy.data

[FILTER]
    Name parser
    Match dummy.*
    Parser json
    Key_Name element
    Nest_Under parsed

[OUTPUT]
    Name stdout
    Match *

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Testing
Before we can approve your change; please submit the following in a comment:

• Example configuration file for the change
• Debug log output from testing the change
  fluentbit-debug.txt

• Attached Valgrind output that shows no leaks or memory corruption was found
  fluentbit-valgrind.txt

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• [N/A] Run local packaging test showing all targets (including any new ones) build.
• [M/A] Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• Documentation required for this feature
  pipelines: filters: parser: Add a nest_under parameter fluent-bit-docs#1543

Backporting

• [N/A] Backport to latest stable release.

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.