feat: add session affinity support

[chakravardhan]

What type of PR is this?
/kind feature

What this PR does / why we need it:

This PR implements support for Nginx session affinity annotations, mapping them to GKE's GCPBackendPolicy

Supported Annotations:

• nginx.ingress.kubernetes.io/affinity: Maps cookie to GENERATED_COOKIE.
• nginx.ingress.kubernetes.io/session-cookie-max-age: Maps to cookieTtlSec.

Dependencies

• Stacked on feat: Implement standard CORS support #303: Relies on the basic infrastructure.

Verification

• Unit Tests: Added TestGCEFeature in gce_converter_test.go

Limitations / Future Work

• Affinity Mode: Strict "persistent" session affinity (strong affinity) support will be added later when the new GKE Gateway CRDs supporting this feature are available and integrated.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

[linux-foundation-easycla]

• ❌ - login: @chakravardhan / name: Chakri Dudyala . The commit (ace80fe, db97ffb) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please submit a support request ticket.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: chakravardhan
Once this PR has been reviewed and has the lgtm label, please assign stevenjin8 for approval. For more information see the Code Review Process.

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[k8s-ci-robot]

Hi @chakravardhan. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[Stevenjin8]

@chakravardhan not going look at the these PRs until the CORS on is merged.

[k8s-ci-robot]

PR needs rebase.

Details

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-sigs/prow repository.

[spencerhance]

/cc