out_es: custom http headers

[AndrewChubatiuk]

Added ability to set custom HTTP headers for Elasticsearch output

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Enter [N/A] in the box, if an item is not applicable to your change.

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

• Example configuration file for the change

[INPUT]
    name              tail
    path              /var/lib/docker/containers/**/*.log
    path_key         path
    multiline.parser  docker, cri
    Parser docker
    Docker_Mode  On

[INPUT]
    Name     syslog
    Listen   0.0.0.0
    Port     5140
    Parser   syslog-rfc3164
    Mode     tcp

[SERVICE]
    Flush        1
    Parsers_File parsers.conf

[Output]
    Name es
    Match *
    host victorialogs
    port 9428
    compress gzip
    path /insert/elasticsearch
    header AccountID 0
    header ProjectID 0
    header VL-Stream-Fields path
    header VL-Msg-Field log
    header VL-Time-Field @timestamp

• [N/A] Debug log output from testing the change

• [N/A] Attached Valgrind output that shows no leaks or memory corruption was found

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.
• [N/A] Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• Documentation required for this feature

fluent/fluent-bit-docs#1390

Backporting

• 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.

[AndrewChubatiuk]

Hey @PettitWesley @edsiper
Could you please help with a review?