Add Time_System_Timezone parser option docs

[braydonk]

This PR adds documentation for the proposed Time_System_Timezone configuration option.

Related PR: fluent/fluent-bit#8164

[patrick-stephens]

@braydonk I presume we don't hit the usual issue with timezones not set in container images? What's the process it uses to get the default timezone (e.g. across OS/containers)?

[braydonk]

Under the hood the feature uses the mktime function from libc, which under the hood uses the POSIX TZ environment variable. This should work on all platforms Fluent Bit supports (including Windows in my testing), but I'm not sure about containers. I don't know if there is anything that Fluent Bit should be expected to do if the TZ env var isn't set properly though.

Should I add this to the table entry here in the docs? Specifically I'd mention the TZ environment variable. EDIT: Decided to add that.

[patrick-stephens]

Yeah, it's a known issue sometimes that TZ may not be set or imported from the host, etc.

[braydonk]

The feature was merged in fluent/fluent-bit#9006. This PR should be good to be merged now.

[patrick-stephens]

Is it in a release @braydonk, that's the normal gate rather than just a merge even though the label suggests otherwise :)