Provide ThresholdSha256Fulfillment.add_subfulfillments()?

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

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

currently:

```python
threshold_sha256.add_subfulfillment(alice_ed25519)
threshold_sha256.add_subfulfillment(bob_ed25519)
```

proposal:

```python
threshold_sha256.add_subfulfillments(alice_ed25519, bob_ed25519)
```

Start by locating ThresholdSha256Fulfillment.add_subfulfillment() and any tests covering subfulfillment handling. Check how multiple arguments should be accepted and verify that add_subfulfillments(alice_ed25519, bob_ed25519) produces the same result as calling add_subfulfillment() twice.

Written by the indexing model from the issue text.