Add checkbox for web based commit signing

What does this MR do and why?

This is a first MR in my implementation plan that you can find here: #580999 (comment 2960767445)

This MR introduces a reusable Web-based Commit Signing Checkbox component and refactors the Group Inheritance Popover to be shared across the application.

Key changes

1. New Web-based Commit Signing Checkbox - Creates a new Vue component (checkbox.vue) that allows users to enable/disable web-based commit signing with proper group inheritance indicators
2. Refactored Group Inheritance Popover - Moves the GroupInheritancePopover component from branch rules to a shared location (vue_shared/components/settings/) and makes it more generic by:
   • Converting injected dependencies to props (hasGroupPermissions, groupSettingsRepositoryPath)
   • Removing branch-specific i18n strings and using generic ones
   • Making it reusable across different settings contexts
3. Updated all consumers - Updates all existing usages of GroupInheritancePopover in:
   • Branch rules protection components
   • Branch rule settings
   • Protection toggle components

Why these changes?

These components will be used in repository settings at both project and group levels to configure web-based commit signing, with proper inheritance indicators when settings are configured at the group level.

References

• Add settings UI to allow groups or projects to ... (#580999 - closed)
• design discussion: #580999 (comment 2941225528)+

Screenshots or screen recordings

Before	After
n/a
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How to set up and validate locally

1. Visit http://localhost:6006/?path=/docs/vue-shared-web-based-commit-signing-checkbox--docs to check the checkbox component
2. Visit http://localhost:6006/?path=/docs/vue-shared-settings-group-inheritance-popover--docs to check the popover component
3. Go to http://gdk.test:8080/groups/flightjs/-/settings/repository#js-protected-branches-settings and create a Protected branch
4. Go to http://gdk.test:8080/flightjs/Flight/-/settings/repository#branch-rules and check the lock icon with popover on a rule that comes from the group level

MR acceptance checklist

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Related to #580999 (closed)