strkey: Extract strkeys into a separate package to leverage `xdr` package.

[Shaptic]

I was suggesting you move this new code into a strkeys package, so that the implementations of different strkeys is independent from the strkey base package. As a bonus this would allow you to import xdr into strkeys.

Originally posted by @leighmcculloch in https://github.com/stellar/go/pull/4279#discussion_r829359421