Pid namespace support

[thebsdbox]

Description

This PR updates the action protos with a pid option of type string, following the --pid flag for the docker engine. The most common use-case is to pass the string host to the pid option which will allow the action to run in the same process ID namespace as the host itself.

Why is this needed

This adds the capability to both cleanly kexec and reboot as an action.

Fixes: 👉 (an already merged kexec action on the tinkerbell action hub)

How Has This Been Tested?

This has been tested by running a workflow with the kexec action

How are existing users impacted? What migration steps/scripts do we need?

If this isn't used then it defaults to pid: container (or the runtime default)

Checklist:

I have:

• updated the documentation and/or roadmap (if required)
• added unit or e2e tests
• provided instructions on how to upgrade

[codecov]

Codecov Report

Merging #436 (5a8ea61) into master (5752ec8) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #436      +/-   ##
==========================================
+ Coverage   34.71%   34.73%   +0.02%     
==========================================
  Files          47       47              
  Lines        2892     2893       +1     
==========================================
+ Hits         1004     1005       +1     
  Misses       1796     1796              
  Partials       92       92              

Impacted Files	Coverage Δ
db/workflow.go	29.20% <100.00%> (+0.14%)	⬆️

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[markjacksonfishing]

/lgtm
Also tested this yesterday