Add support for operator in filter bar

What does this MR do?

This MR adds support for Operator Dropdown in the filter search bar.

Issue ref: #18059 (closed)

Todo:

• Test this MR everywhere. Right now developing for Issue list page but since code is shared, it should work everywhere.
• Integration with backend

The URL would look something like author=%3Dfoo => key: 'author', operator: '=', value: 'foo'

• Bug: Selection with a mouse click on the value dropdown does not work.
• Bug: When adding multiple filters, the operator dropdown opens when value dropdown should open.
• Bug: When selecting values with a mouse click, the next dropdown does not open.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• 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