Make namespace setup/cleaner a KTF utility

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Problem Statement

[setup()](https://github.com/Kong/gateway-operator/blob/bdd220ef35db150836d39a79c22a2554e8faa859/test/integration/util_test.go#L16-L35) is a generic function to set up a namespace and provide a function to clean up it and entities. It lives in gateway-operator; we'd like to use it in KIC also.

### Proposed Solution

- Migrate `setup()` into KTF.
- Optionally add the ability to spawn and return multiple namespaces. Several KIC tests use these but the method for handling them is a bit janky.

### Additional information

_No response_

### Acceptance Criteria

- [ ] tests can import a KTF package that provides a function to create a namespace and cleaner function in a KTF environment.
- [ ] the relevant TODO comments in KIC and KGO have been cleaned up

Start with setup() in test/integration/util_test.go, then inspect the KTF package structure and the relevant TODO comments in KIC and KGO. Determine how the namespace creator and cleaner should be exposed, including optional support for multiple namespaces. Done means tests can import the KTF utility and the TODO comments are removed.

Written by the indexing model from the issue text.