Change address purposes to strings

[modular-magician]

I'm not sure if this actually has any user impact so there's no changelog entry, but we won't block users wanting to adopt new purposes. New ones come up too frequently for me to want to track possible values anymore.

Interestingly, comparing to https://cloud.google.com/compute/docs/reference/rest/beta/addresses and https://cloud.google.com/compute/docs/reference/rest/beta/globalAddresses, we support some purposes that don't appear there and don't support purposes that do. That's an even more clear sign we should get out of the business of tracking these.

The c.g.c page additionally doesn't disambiguate between what's available in global / regional addresses, and some of them are external so I removed the internal warning in both types.

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Derived from GoogleCloudPlatform/magic-modules#4681