victoria-metrics: support to send logs to syslog server

[wasim-nihal]

Describe Your Changes

Changes to support sending of logs to syslog server. See #5903.

This change uses the Rackspace syslog package (https://github.com/RackSec/srslog) to send logs to syslog server. The standard log/syslog package is not implemented for windows that cause the windows builds to fail. This package has the support for TLS+TCP communication with syslog server.

The standard syslog documentation mentions the below points:

Bugs
This package is not implemented on Windows. As the syslog package is frozen, Windows users are encouraged to use a package outside of the standard library. For background, see https://golang.org/issue/1108.

This package is not implemented on Plan 9.

Note: Currently, this change does not support sending logs to syslog with "TCP+TLS". Once it's decided to use this syslog package or to maintain one within the Victoria Metrics' `/lib', the TLS support can be implemented.

See previous implementation by @khushijain21

Checklist

The following checks are mandatory:

• My change adheres VictoriaMetrics contributing guidelines.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 22 lines in your changes are missing coverage. Please review.

Project coverage is 56.84%. Comparing base (8aaa828) to head (2103a8c).
Report is 633 commits behind head on master.

Files	Patch %	Lines
lib/syslog/syslog.go	0.00%	17 Missing ⚠️
lib/logger/logger.go	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6204      +/-   ##
==========================================
- Coverage   60.37%   56.84%   -3.53%     
==========================================
  Files         411      528     +117     
  Lines       76609    72474    -4135     
==========================================
- Hits        46253    41201    -5052     
- Misses      27794    28373     +579     
- Partials     2562     2900     +338     

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