Add buffer configuration to external auth location config

[Eddman]

What this PR does / why we need it:
If the External Auth service returns for instance cookies that are bigger then 4Kb the request fails.

This happens whenever external auth service returns cookies and other headers that exceed the default proxy buffer (4Kb). The configmap option proxy-buffer-size is completely ignored and its proxy_buffer_size equivalent is not propagated to the auth location.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged):
fixes #2427

Special notes for your reviewer:
no

[aledbf]

@Eddman please run make code-generator and push the changes

[Eddman]

@aledbf Done.

[codecov-io]

Codecov Report

Merging #2428 into master will increase coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2428      +/-   ##
==========================================
+ Coverage   40.91%   41.06%   +0.15%     
==========================================
  Files          74       74              
  Lines        5248     5248              
==========================================
+ Hits         2147     2155       +8     
+ Misses       2809     2801       -8     
  Partials      292      292

Impacted Files	Coverage Δ
internal/file/bindata.go	60.17% <ø> (ø)	⬆️
internal/watch/file_watcher.go	84.61% <0%> (+3.84%)	⬆️
cmd/nginx/main.go	26.2% <0%> (+4.82%)	⬆️

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[aledbf]

/lgtm

[aledbf]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, Eddman

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• OWNERS [aledbf]

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[aledbf]

@Eddman thanks!