Add custom labels to get topic tree

[clfkenny]

What does this PR do?

get_topic_tree function currently does not accept custom_labels as a parameter whereas the other visualization methods do. Therefore this change aims to fix that issue so custom labels can be accepted and make the visualization more intuitive.

I really enjoyed looking through the codebase here. My first PR here so still learning the project structure, would appreciate any feedback. Thanks!

Fixes # https://github.com/MaartenGr/BERTopic/issues/2123

Before submitting

• [ ] This PR fixes a typo or improves the docs (if yes, ignore all other checks!).
• [x] Did you read the contributor guideline?
• [x] Was this discussed/approved via a Github issue? Please add a link to it if that's the case.
• [ ] Did you make sure to update the documentation with your changes (if applicable)?
• [ ] Did you write any new necessary tests?