Provide ThresholdSha256Fulfillment.add_subfulfillments()?

[sbellem]

Being able to pass multiple fulfillments at once would be convenient. E.g., given:

alice_ed25519 = Ed25519Fulfillment(public_key=alice.verifying_key)
bob_ed25519 = Ed25519Fulfillment(public_key=bob.verifying_key)
threshold_sha256 = ThresholdSha256Fulfillment(threshold=2)

currently:

threshold_sha256.add_subfulfillment(alice_ed25519)
threshold_sha256.add_subfulfillment(bob_ed25519)

proposal:

threshold_sha256.add_subfulfillments(alice_ed25519, bob_ed25519)