E2E: Add skip_contact Option to Purchase Flows

What does this MR do and why?

MR acceptance checklist

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Resolves #435165 (closed) and #435060 (closed)

This MR updates the methods in qa/qa/flow/purchase.rb to accept a skip_contact parameter. When set to true, this will skip filling in the contact information for the customer during the purchase flow. This is to account for test cases that use existing billing accounts (ex: when a customer had already made a previous purchase earlier in the test), since they are no longer allowed to update their contact info during the purchase flow.

Please see the associated CustomersDot MR that introduced similar functionality

This will allow us to dequarantine the following tests from fast quarantine:

• qa/specs/features/ee/browser_ui/11_fulfillment/purchase/purchase_ci_spec.rb:55
• qa/specs/features/ee/browser_ui/11_fulfillment/purchase/purchase_ci_spec.rb:82
• qa/specs/features/ee/browser_ui/11_fulfillment/purchase/upgrade_group_spec.rb:58

How to set up and validate locally

These tests can only run against Staging Canary and Staging. To run these tests locally against Staging, you will need to export the following ENV vars:

export GITLAB_USERNAME=gitlab-qa
export GITLAB_PASSWORD=<Found in GitLab-QA 1Password>
export GITLAB_QA_USER_AGENT=<Found in GitLab-QA 1Password>
export GITLAB_ADMIN_USERNAME=gitlab-qa-bot
export GITLAB_ADMIN_PASSWORD=<Found in GitLab-QA 1Password>
export GITLAB_QA_ACCESS_TOKEN=<Found in GitLab-QA 1Password>
export GITLAB_QA_ADMIN_ACCESS_TOKEN=<Found in GitLab-QA 1Password>
export QA_GITLAB_URL=https://staging.gitlab.com

Then, run one of the tests from the qa directory, ex:

bundle exec rspec qa/specs/features/ee/browser_ui/11_fulfillment/purchase/purchase_ci_spec.rb