vlinsert: journald ingestion support

[AndrewChubatiuk]

fixes https://github.com/VictoriaMetrics/VictoriaMetrics/issues/4618

~requires https://github.com/valyala/gozstd/pull/64 to fix ZSTD compression for future systemd support~ (merged)

Added support of HTTP headers and ZSTD compression as well, though it's not yet supported in systemd-journal-upload.

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[AndrewChubatiuk]

@f41gh7 updated PR, please take a look

[f41gh7]

Thanks for contribution!