External Account API: initial SEP draft (requesting SEP-58)

Submits the External Account API draft that has been iterated in
discussion #1902 since 2026-03-19. Filename uses the README's
sep_{shorttitle}.md draft convention; the preamble claims SEP: 0058
based on discussion-thread precedence, but the author will accept
whatever number maintainers assign.

Note: SEP-0058 was concurrently assigned in PR #1933 to Leigh
McCulloch's "Contract Build Reproducibility" draft. This submission
predates that one by ~6 weeks of community discussion (#1902 since
March; #1923 since May). Coordination with @leighmcculloch is
underway out of band. Either author is happy to take a different
number — flagging the situation for maintainers.

Spec highlights (v0.4.0):
- Standard API for provisioning inbound receiving instruments
  (crypto deposit addresses, virtual bank accounts, bank credentials)
  bound to an authenticated Stellar wallet.
- Authentication via SEP-10 (G.../M...) and SEP-45 (C...).
- v0.4.0 adds currency to crypto_address offerings/requests/responses
  and to the idempotency key, motivated by real anchor deployments
  (e.g. Bridge) offering multiple currencies on the same chain_id.

Discussion: https://github.com/orgs/stellar/discussions/1902

Author: antb123