Migration: deprecate borderless-accounts endpoint, and provide account-detail and balance-statement endpoints.

[jayaddison]

It's likely going to make sense to introduce some datatypes (dataclasses perhaps?) for the result objects in the nearish future; for now though, make the endpoints available.

My understanding is that the DeprecationWarning is unlikely to appear in many downstream applications at runtime, since it's ignored by default; it's most likely to appear when test suites run.

Resolves #24.

[marksteward]

Looks good from a read! This still requires SCA, is that done differently to the old endpoints, or are we still going to need to find an alternative to private keys? We obviously need to make this change, but I'm just wondering if this takes away the potential imminent doom from #24.

[jayaddison]

Looks good from a read! This still requires SCA, is that done differently to the old endpoints, or are we still going to need to find an alternative to private keys? We obviously need to make this change, but I'm just wondering if this takes away the potential imminent doom from #24.

Thanks! My sense - and I'll check again tomorrow because I already had to remind myself - is that for business accounts, retrieving statements using signature-based authentication is still OK. That's not something I've confirmed with Wise, though.