Add a custom commit message when applying a suggested change (FE)

What does this MR do?

This MR adds the ability to define a custom commit message when applying a suggested change.

This MR is part of a number of iterations towards delivering #25381 (closed).

Note: changes in this MR are behind the suggestions_custom_commit feature flag.

To enable the feature flag:

1. Run rails c in the project root.
2. Run Feature.enable(:suggestions_custom_commit) in the rails console.

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry (not needed)
• Code review guidelines
• Merge request performance guidelines
• Style guides

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Related to #25381 (closed)