SEP-10: Add support for contracts (storage bound)

[leighmcculloch]

What

Add support for contracts to SEP-10 using storage to store a fixed key.

Why

So that contract accounts can authenticate with SEP-24, SEP-30, etc.

State

This PR is a early draft and idea for how SEP-10 could be modified.

cc @JakeUrban @tomerweller @philipliu @reecexlm @mtwtfss

Related:

• https://github.com/stellar/stellar-protocol/pull/1494

[leighmcculloch]

Closing in favor of proposing:

• https://github.com/stellar/stellar-protocol/pull/1494