Draft: POC: use flag_shih_tzu gem for storing permissions for customizable roles

What does this MR do and why?

This a proof of concept to evaluate to use of flag_shih_tzu gem as suggested in #352891 (comment 1059561579).

Customizable roles currently support a single permission: download_code. However, we plan on making most of our permissions defined in the policies files customizable. As a result, MemberRole model will have lot's of boolean columns. flag_shih_tzu allows the use of ActiveRecord bit fields. It helps reducing the number of columns

This POC tries to achieve 2 goals:

1. introduces flag_shih_tzu gem and bit fields in the MemberRole model
2. modifies Preloaders::UserMemberRolesInProjectsPreloader to load all customizable permissions (not just download_code), and calculates the resulting effective permissions based on them

Pros and cons of the gem

Pros:

• optimized storage use
  • with flag_shih_tzu, using an integer column can store up to 32 flags
  • the same amount of storage can store only 4 boolean column in postgres
• we can avoid add new migrations for each new customizable permission

Cons:

• the gem does not meet the quality standards for external libraries
• adds an additional layer of logic to manage integer column as bit field

Related to #375575 (closed).

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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