Add for_associations selector for anonymising related records

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It is rare that records need to be anonymised in isolation, and this makes it easy to configure the associations on a model which should be anonymised at the same time. This should greatly simplify finding and anonymising all records for a given subject across large, complex database structures

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Needs #106 to be approved and merged for the rubocop checks to pass

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Changed to a draft because it's not actually fully ready yet, I just want to seek opinions early

I know I need to handle circular references, which should just be a case of checking if anonymised? is true at the beginning of anonymise!

@xmjw pointed out that I should check that we won't run into potential issues with stack depth which is a good shout and I will look into that

I would also like thoughts on the return value. I wanted to ensure that if there are no associations being anonymised, that we don't change what we return for compatibility, but I wasn't sure whether to have some kind of MergedResult that includes information about all the changes, or to use a hash with the model name as the key and the result as the value, so I just did the simple thing for now and wanted to ask for thoughts from others

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See https://github.com/orgs/gocardless/discussions/505 for more info on why I think we should make this change