SEP-10: Add support for contracts (Soroban Auth bound)

[leighmcculloch]

What

Add support for contracts to SEP-10 using Soroban Auth.

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 @dmkozh

Related:

• https://github.com/stellar/stellar-protocol/pull/1492

[github-actions[bot]]

This pull request is stale because it has been open for 30 days with no activity. It will be closed in 30 days unless the stale label is removed.

[leighmcculloch]

Closing to approach the change as a new SEP, largely contingent on:

• 1534