Add a new system config 'enable_jit'

[fujimotos]

Which issue(s) this PR fixes:

N/A

What this PR does / why we need it:

This adds a new option to enable Ruby JIT in worker processes.

Here is an example configuration:

<system>
  workers 3
  enable_jit true
</system>

The exact JIT strategy (in particular, mjit or yjit) will be chosen
by Ruby as it sees fit. See the following ticket for details:

https://bugs.ruby-lang.org/issues/18349

Docs Changes:

fluent/fluentd-docs-gitbook#422

Release Note:

Add a new system configuration enable_jit

[fujimotos]

The good news is that Ruby JIT made Fluentd 10%-20% faster on my Ubuntu
22.04. Here is the summary of my experiments (with tail + ltsv parser):

The not-so-good news is the performance varies a lot depending
on configurations. For example, using parser_json or
parser_apache resulted in no performance gain. So use it wisely.

[daipom]

Thanks for the fix!
I'm not sure about Ruby JIT, but I don't see any problem with this fix.

[kenhys]

LGTM.

[ashie]

LGTM

It might be better that add a command line option for this, we can add it by another pull request if it's really required.

[k0kubun]

Here is the summary of my experiments (with tail + ltsv parser)

@fujimotos Do you happen to have the benchmark script still? I'm interested in running the same thing if possible.