Support cumulative permissions on explicit sessions

[ScreamingHawk]

Using sessions with cumulative permissions requires appending a valid incrementUsageLimit call to the payload. The flow is now:

const payload = { /* ... */ }
const increment = await sessionManager.prepareIncrement(wallet.address, chainId, payload.calls)
if (increment) payload.calls.append(increment)
// Sign and send as normal

Note that perpareIncrement will return null if no increment is required. This function/flow can safely be used for all types of sessions and permissions.