Add feature specs for Opt-in Identity Verification

Context

Currently, users go through identity verification as part of the signup process. Moving forward, we want to allow users¹ that have already started using Gitlab to go through an opt-in identity verification process where they can verify a phone number in exchange for in-app benefits (e.g. ability to run more pipelines, create more groups, etc.).

See https://gitlab.com/groups/gitlab-org/modelops/anti-abuse/-/epics/32+ for more info.

¹ Limited to users that did not go through phone number and/or credit card verification during signup.

What does this MR do?

Implements part of https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/667.

This MR adds initial feature specs for Opt-in Identity Verification. Some cases (see checklist) are not covered because they are dependent on MRs that are still in flight.

MR acceptance checklist

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Before	After

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