Adding onBlur prop

[simonychuang]

Adding the onBlur prop that is supported by the react-ace editor to be used in the new sigma rules editor. Related PR here: https://github.com/Graylog2/graylog-plugin-enterprise/pull/4094

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Refactoring (non-breaking change)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.