Move CustomDist logic to its own file

[ricardoV94]

Description

The logic of CustomDist is getting pretty large. After #7239 I would refactor it into it's own file (and same for tests).

[markgreene74]

Hi 👋 I am here at the hackathon at Pydata London 2024. I would like to give this a try.

[Hemant19870601]

Hi Mate.. Even I am in same area without laptop. If you want any help or ok to work together, Give me shout.

[markgreene74]

@Hemant19870601 and I are working on this branch: https://github.com/markgreene74/pymc/tree/issue-7247-move-custom-dist-logic

Made good progress today! We should be able to finish it soon and raise a PR.