feat(cmd/serve): add in default-shutdown-timeout flag to increase shutdown timeout on http server shutdown

[ottingbob]

Under the hood ory/oathkeeper uses https://github.com/ory/graceful to shutdown the HTTP proxy. The graceful package provides a constant called DefaultShutdownTimeout which sets the timeout to be 5 seconds.

We have an application that can take up to 60 seconds to finish processing requests and we need a solution to be able to change this shutdown time in order for those requests to complete successfully.

As a result, we need a way to configure oathkeeper to bump up that timeout and the implementation has a flag that will be able to override the global variable set in the graceful package. Although this does not seem like the most ideal solution it provides a way for us to change that value.

Curious to hear suggestions on what may be an alternative approach to get a similar outcome & would be happy to try and make the changes needed for any feedback to help enable that.

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
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  vulnerability. If this pull request addresses a security vulnerability, I
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  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

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[ottingbob]

Curious to hear suggestions on what may be an alternative approach to get a similar outcome & would be happy to try and make the changes needed for any feedback to help enable that

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.81%. Comparing base (8fc9b7a) to head (2d97c76).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1162      +/-   ##
==========================================
- Coverage   77.90%   77.81%   -0.09%     
==========================================
  Files          80       80              
  Lines        3929     4044     +115     
==========================================
+ Hits         3061     3147      +86     
- Misses        595      617      +22     
- Partials      273      280       +7     

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