Move pipeline file extension check to YAML checker -> question regarding a generic pipeline checker

In the UI layer there's a check of whether the extension of the pipeline file provided to httomo is "valid" (currently, YAML and python scripts are supported), and a `ValueError` is raised if an unsupported file extension is given: https://github.com/DiamondLightSource/httomo/blob/ae5610a83a691436e0e7deff3a71c2af3f89568e/httomo/ui_layer.py#L52-L56

It would make sense to have this error occur prior to httomo beginning to construct the pipeline.

There's some more though necessary I think though. The "YAML checker" is exactly that: it checks pipeline files that are written in YAML, but doesn't check pipeline files written in python. Moving this file extension check to the YAML checker is a bit odd: it's a checker of YAML files, but it'll accept a python pipeline despite not checking it?

It seems like maybe there needs to be a generic "pipeline checker" in order to validate a pipeline in any format, and then the file extension check can be done in the generic pipeline checker.

Whether or not we want to deal with creating a generic checker is one of the main questions here.