Support signed payload revoke signer sponsorship

[chrisx9z]

Summary

• Adds signerKeyTypeEd25519SignedPayload parsing in convertXDRSignerKeyToObject.
• Allows Operation.revokeSignerSponsorship to build revoke-signer sponsorship ops for P-address signers.
• Adds a round-trip test covering build, XDR decode, and Operation.fromXDRObject output.

Fixes #966.

Verification

• corepack yarn install --ignore-scripts
• corepack yarn mocha test/unit/operations/classic_ops_test.js --grep "signed payload signer|revokeSignerSponsorship" --require @babel/register --require ./test/test-helper.js
• corepack yarn eslint -c ./config/.eslintrc.js src/operation.js src/operations/revoke_sponsorship.js
• node --check src/operation.js
• node --check src/operations/revoke_sponsorship.js
• node --check test/unit/operations/classic_ops_test.js
• git diff --check

Note

• Running ESLint over the full test/unit/operations/classic_ops_test.js file reports existing style violations unrelated to this change, so source files were linted directly and the changed test path was verified with the focused mocha run and syntax check.