Switch to new validation process for CodeOwners when feature is enabled

What does this MR do and why?

Related to Inaccessible Code Owner validation (#429261 - closed)

Switch to new validation process for CodeOwners when feature is enabled

This is the first MR in a series of MRs to switch to a new validation process for Code Owners.

This MR introduces a new wip feature flag named accessible_code_owners_validation which will switch between the old validation process and the new one.

To keep the scope of this change as focused as possible it is limited to changes in the CodeOwners::File only.

A following change will add logic to the CodeOwners::OwnerValidation::Process to validate accessible code owners.