SPIKE: CAP-71 SDK support — expose delegate_account_auth

[sisuresh]

SPIKE used to e2e-test CAP-71 (Auth delegation) end-to-end against a Quickstart Protocol 27 build (with stellar-core PR #5209 = CAP-83 + CAP-71 enabled via `--enable-next-protocol-version-unsafe-for-production`).

What this changes

• Pins `soroban-env-{common,guest,host}` and `stellar-xdr` to the CAP-71 commit:
  • `rs-soroban-env` @ `7fb0a840…` (PR Bump version to 23.1.0 #1607, merged) — adds the new `address.delegate_account_auth` / `get_delegated_signers_for_current_auth_check` host fns.
  • `rs-stellar-xdr` @ `a749b69b…`, `features=["cap_0071"]` — adds `SOROBAN_CREDENTIALS_ADDRESS_V2`, `…_WITH_DELEGATES`, `HashIdPreimageSorobanAuthorizationWithAddress`, `SorobanDelegateSignature`.
• Drops the `curr` feature on `stellar-xdr` (the new per-CAP-feature layout has no `curr` module; everything is at the crate root).
• Replaces `use stellar_xdr::curr as stellar_xdr;` aliases with crate-root imports.
• Adds `Address::delegate_account_auth(&self)` — wraps `env.delegate_account_auth(AddressObject)` so a custom-account contract's `__check_auth` can call `delegate.delegate_account_auth()` to hand off auth (CAP-71-01).

How it was used

A 13-line custom-account contract was built against this SDK and deployed to a Quickstart p27 node; a `SOROBAN_CREDENTIALS_ADDRESS_WITH_DELEGATES` auth entry (with a single `SorobanDelegateSignature`) was submitted; the tx returned `SUCCESS` from RPC and was ingested by Horizon without error. (Companion testing on `go-stellar-sdk`, `stellar-rpc`, and `stellar-horizon` is tracked in those repos' SPIKE PRs.)

Notes for a real PR

• Skipping the `get_delegated_signers_for_current_auth_check` wrapper for now (returns `VecObject` → `Vec` needs a real conversion path).
• The `stellar-xdr` version constraint had to drop from `=26.0.1` to `=26.0.0` to match the rev's self-reported version.
• Many `use stellar_xdr::curr as stellar_xdr;` aliases were removed mechanically. A real PR should consolidate to a single re-export.