Customizable commit messages for applied suggested changes

What does this MR do?

This MR adds the capability to define a custom commit message, which is used when a change suggestion on a merge request is applied. The configuration is done on a project settings level and supports free text with GitLab variables. The old hardcoded commit message is used as a fallback option.

The suggest changes feature is extremely helpful, but as soon as commit message conventions (e.g. conventional commit) are used and enforced (e.g. GitLab EE commit message hooks or GitLab-CI), it becomes cumbersome or unusable.

Closes #13086 (closed), Addresses #25381 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team