Added client attribution to SEP-10 helpers

[JakeUrban]

resolves #668

Added client attribution matching the implementation in the Python SDK.

[Shaptic]

Looks like a straightforward translation of the Python version, but we are of course presuming that that one behaves correctly. Can you confirm that it does what SEP-10 says it should do? I see this section in the SEP but am not 100% sure that's what we're adding here.

[JakeUrban]

Can you confirm that it does what SEP-10 says it should do?

Its used in production for several wallet / anchor pairs and haven't heard of any issues, so I think its stable.

I see this section in the SEP but am not 100% sure that's what we're adding here.

Yea thats the right section. The idea is that SEP-10 verifies the user and anchor as account holders, but the anchor doesn't know which wallet is being used. If the wallet hosts their own stellar.toml file and signs the challenge with their SIGNING_KEY, the anchor can verify that a specific wallet is being used.