Order should support multiple signatures

[juniset]

The IAccount interface is designed to leverage account abstraction and not make any assumption on the type and size of the signatures that are valid for an account. In particular, an account can have multiple signers as is the case in the ArgentAccount and that is why signatures must be validated using IAccount.isValidSignature.

The Order struct today assumes a single signature (i.e. an array of 2 felt)

starknet-contracts/zigzag.cairo

Lines 87 to 88 in c8b8068

	member sig_r: felt
	member sig_s: felt

. This should be updated to support an array of arbitrary length.