Save API fuzzing configuration

What does this MR do?

Implements the save functionality in the API fuzzing form.

How to test this?

1. Enable the :api_fuzzing_configuration_ui feature flag:

echo "Feature.enable(:api_fuzzing_configuration_ui)" | rails c

2. Pull 299234-api-fuzzing-config-mutation (!53883 (merged)) into this branch

3. Browse to the API fuzzing configuration form at /:namespace/:project/-/security/configuration/api_fuzzing

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry (behind a feature flag)
• [-] 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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Related to #299127 (closed)