Switch resolve sequence number to use `/v1/accounts/ADDR/keys/KEY_INDEX` endpoint

[jribbink]

trafficstars

We should stop querying the whole account when resolving the sequence number in the preflight request before a transaction is sent.

Latency-wise, this can be extremely costly as wallets will have massive key counts on their proposer system accounts.

Example request: GET https://rest-mainnet.onflow.org/v1/accounts/0x34ac358b9819f79d/keys/0