Add support for setting pids_limit in docker plugin config.

[shishir-a412ed]

Currently, Nomad doesn't have a way to enforce pids_limit in containers, unless set by user in their job spec.
If a malicious user launches a container with a fork-bomb, that could exhaust entire nomad client host pids.

This PR will allow the operator to enforce pids_limit at Nomad client plugin config. User can still override that pids_limit in their job spec by setting pids_limit however it must be less than the limit defined on nomad client plugin config.

[lgfa29]

Thanks @shishir-a412ed this LGTM!

I pushed a few commits to add a CHANGELOG entry and reworded the docs a bit. I also added a new test case to check if the task-level config overrides the plugin config.

[github-actions]

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