Implement `__getattr__` for `Prior` class

[williambdean]

I would be nice to be able to dot into Prior class and get the parameter. For instance dist.mu. Would be similar or same implementation to this:

https://github.com/pymc-labs/pymc-marketing/blob/96848210a982279c50529ee828191cc9725b1c7f/pymc_marketing/prior.py#L396-L398

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This issue was transferred from pymc-labs/pymc-marketing#1352