Auth snippet

[abaruni]

What this PR does / why we need it: Add the ability to specify a custom snippet to use in conjunction with external auth. e.g. adding custom headers to the auth request. Please see #3274

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #3274

Special notes for your reviewer:

[k8s-ci-robot]

@abaruni: GitHub didn't allow me to assign the following users: diazjf.

Note that only kubernetes members and repo collaborators can be assigned.
For more information please see the contributor guide

In response to this:

/assign @diazjf

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[diazjf]

@abaruni needs e2e tests as well. See https://github.com/kubernetes/ingress-nginx/blob/master/test/e2e/annotations/snippet.go as a sample and make sure to test with multiple lines.

Once added you can test by running FOCUS=<framework-name> make e2e-test. You can add the test to https://github.com/kubernetes/ingress-nginx/blob/master/test/e2e/annotations/auth.go

Also needs some documentation.

[diazjf]

Also rebase with master to resolve the tests which are currently failing.

[diazjf]

Updated with a few non-critical things to add.

[diazjf]

Also make sure to squash into 1 commit.

[aledbf]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [aledbf]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[aledbf]

@abaruni thanks!