Get notarization credentials for xcrun notarytool

Returns the caller's own ASC key material (key id, issuer id, and the .p8) for xcrun notarytool submit --key <.p8> --key-id <keyId> --issuer <issuerId> --wait on a Mac build VM. This is the only endpoint that returns the .p8 — a deliberate, owner-only exposure of the caller's own key; the credential status endpoint never leaks it. 409 if no ASC key is configured.