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

• [x] 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.
• [ ] I have read the security policy.
• [x] I confirm that this pull request does not address a security vulnerability. If this pull request addresses a security vulnerability, I confirm that I got the approval (please contact security@ory.sh) from the maintainers to push the changes.
• [ ] I have added tests that prove my fix is effective or that my feature works.
• [ ] I have added or changed the documentation.

[CLAassistant]

All committers have signed the CLA.

[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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 77.81%. Comparing base (8fc9b7a) to head (2d97c76). Report is 6 commits behind head on master.

Additional details and impacted files

```diff @@ 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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