Allow sessionAffinity to be set on gitlab-pages custom domains Service

What does this MR do?

This allows the sessionAffinity option to be set for the gitlab-pages custom domains service. The option is already available for the regular service. This is in pursuit of resolving an issue that the Docs site is experiencing where the site breaks during site deployments.

Related issues

gitlab-com/gl-infra/production-engineering#28026

Author checklist

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Required

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• Merge Request Title and Description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• MR has a green pipeline.
• Documentation created/updated.
• Tests added/updated, and test plan for scenarios not covered by automated tests.
• Equivalent MR/issue for omnibus-gitlab opened.

Reviewers checklist

• MR has a green pipeline on https://gitlab.com/gitlab-org/charts/gitlab.
• Consider downstream impact to the Operator, as per evaluating impact from changes to GitLab chart.