Add `scripts/decomposition/generate-loose-foreign-key` script

What does this MR do and why?

This is internal tooling as referenced in &7249 (comment 816243722).

This adds a helper script to automate loose foreign key swap. Script does:

• create branch
• creates templated migration
• runs migrations to commit db/structure.sql
• updates YMLs for LFK
• updates specs for LFK
• generates LFK spec in a model
• commits
• pushes
• provides MR template

Usage: ./generate-loose-foreign-key.rb [options] <filters...>

    -c, --cross-schema               Show only cross-schema foreign keys
    -n, --dry-run                    Do not execute any commands (dry run)
    -b, --[no-]branch                Create or not a new branch
    -r, --[no-]rspec                 Create or not a rspecs automatically
    -m, --milestone                  Specify custom milestone (current: 14.7)
    -h, --help                       Prints this help

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