test(e2e): import + deploy StarknetEth712Account, verify case-I EVM signature (#927)

Add starkware_accounts::StarknetEth712Account to the privacy test build's build-external-contracts so
it is declarable on devnet. e2e/src/eth712-account-setup.ts deploys + initializes one (EVM key ->
address, ownership signature over the fixed hash, UDC deploy + initialize). The test confirms the
client's Eip712HashSigner produces a CallSet signature the deployed account accepts on-chain via
is_custom_signature_valid (case I).

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