Add ability to add read_code to custom roles

What does this MR do and why?

• download_code and read_code are now separate via #376180 (closed)
• for the first iteration of a customer-facing MVC, we want to provide the ability to customize read_code for a Guest user.
• We previously added the ability to customize download_code but we will be removing that for the customer MVC.
• This is all behind the customizable_roles feature flag
• #20277 (closed)

Database

UserMemberRolesInProjectsPreloader query before this change: https://explain.depesz.com/s/shQU

UserMemberRolesInProjectsPreloader query after this change: https://explain.depesz.com/s/yP4E

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MR acceptance checklist

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