backend/azure: Remove legacy helper/schema dependency

[apparentlymart]

As part of our efforts to remove the large snapshot of the legacy SDK that we've been using, this replaces the uses of that package with our new lightweight backendbase package, which is a collection of helpers that can substitute for most of the parts of the legacy SDK that backends tend to use.

In the long run we're still planning to move all of the state storage backends out to provider plugins, at which point they'll presumably use the modern plugin framework, but in the meantime the legacy SDK is quite a liability because few people know how to maintain it while hopefully this interim backendbase package is easier to maintain (if needed) due to its relative simplicity.

I don't have any test environment for this backend, so I've only tested this through its unit tests. I'd appreciate if a backend maintainer could, along with reviewing this code, also run the acceptance test suite in case there are regressions that the unit tests aren't able to catch. Thanks!