Support cross-region SSM parameter resolution

[mipearson]

Had a case where I was about to copy a bunch of SSM keys to another region and then realised that there was a better way.

Struggled with trying to write tests for this: the existing tests, in their use of mocking the AWS api, will return the same response regardless of which region or SSM key is queried. So this is technically "covered" - but not explicitly tested.

[orien]

Don't forget to add an entry in the change log.