feat(backend): Add the ability to set a proxy for accessing external resources

[hbelmiro]

Resolves #11744
Resolves #9702

Description of your changes:
This PR adds the ability to set a server-scoped proxy for accessing external resources.

Docs

• pipelines: Added proxy documentation website#4073

Notes for reviewers

• To ensure a "fail fast" approach (preferably in compile time), I:
  • Made the proxy configuration mandatory everywhere but in the entry point, which is in the apiserver main function when we create an instance of proxy.Config. The factory method handles the defaults for the unset values.
  • Created a new constructor for argocompiler.workflowCompiler that receives proxy.Config as a parameter. All existing fields are still optional.
    • Theoretically, argocompiler.workflowCompiler.proxyConfig is also optional since argocompiler.workflowCompiler is a struct. But changing that to make it mandatory at compile time would increase the number of changes in this PR too much. We can do that in a follow-up PR if we think it's necessary
  • Created a new interface for the proxy configuration with a private struct that implements it
  • Didn't leverage other existing "Options" structs
• Tests
  • config_test.go tests all combinations possible and their default values when needed
  • End to end tests use Squid as a proxy deployed on Kind
    • The test runs a pipeline that reads a proxy variable. The pipeline succeeds if the variable was set. The pipeline should fail when no proxy is set.

Checklist:

• You have signed off your commits
• The title for your pull request (PR) should follow our title convention. Learn more about the pull request title convention used in this repository.

[google-oss-prow]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[nithin8702]

This is very useful and waiting for this PR to get merged. Thanks

[hbelmiro]

@hbelmiro could you please rebase?

@mprahl done.

[mprahl]

A few minor comments but this overall looks good!

[hbelmiro]

/retest

[hbelmiro]

/retest

[mprahl]

/lgtm

[hbelmiro]

@HumairAK can you take another look?

[nithin8702]

This is much required feature to us as we heavily involved in using corporate proxy. Thanks for this team. We would like to get this feature as early as possible.

[hbelmiro]

/retest

[mprahl]

/approve
/lgtm

[hbelmiro]

/retest

[hbelmiro]

/retest

[nithin8702]

Could someone please approve this PR

@HumairAK @mprahl

[HumairAK]

/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: HumairAK, mprahl

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:

• .github/OWNERS [HumairAK]
• backend/OWNERS [HumairAK,mprahl]

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