Add include_in_schema parameter to headers to allow exclusion from OpenAPI spec

[astraldawn]

Partially addresses #1708

Have the same use case as described by @mnorrsken

We have a header with auth info thats always sent from our API manager to our backend. 
I want to consume this header in my API but I don't want it to show up in the docs.

[codecov]

Codecov Report

Merging #3012 (f34b9e1) into master (6f72a27) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #3012   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          243       244    +1     
  Lines         7419      7431   +12     
=========================================
+ Hits          7419      7431   +12     

Impacted Files	Coverage Δ
fastapi/param_functions.py	100.00% <ø> (ø)
fastapi/openapi/utils.py	100.00% <100.00%> (ø)
fastapi/params.py	100.00% <100.00%> (ø)
...st_tutorial/test_header_params/test_tutorial004.py	100.00% <100.00%> (ø)

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[github-actions]

📝 Docs preview for commit 6264c0d at: https://60621dd1d9d36700acacba00--fastapi.netlify.app

[github-actions]

📝 Docs preview for commit 29c8e57 at: https://60633f575d81a90f6d0b4ef7--fastapi.netlify.app

[github-actions]

📝 Docs preview for commit 2447106 at: https://606340fc6ced6f136b7f432f--fastapi.netlify.app

[github-actions]

📝 Docs preview for commit f34b9e1 at: https://606342b370838e0ea4d4b3e2--fastapi.netlify.app

[astraldawn]

Closing in favour of #3144

[tiangolo]

Thanks for coming back and closing the issue 👍

Sorry for the long delay! 🙈 I wanted to personally address each issue/PR and they piled up through time, but now I'm checking each one in order.