feat: add instant-seal for dev chain

[gregdhill]

Signed-off-by: Gregory Hill gregorydhill@outlook.com

This PR adds the --instant-seal option to the interbtc-parachain binary which automatically generates blocks when new transactions are added to the mempool or when calling the manual-seal RPC:

curl -H "Content-Type: application/json" -d '{"id":1, "jsonrpc":"2.0", "method": "engine_createBlock", "params": [true, true, null]}' http://localhost:9933/

Succeeds #363

[sander2]

Is this working with the clients? The last attempt at instant seal failed on this: paritytech/substrate#10279

[gregdhill]

Is this working with the clients? The last attempt at instant seal failed on this: paritytech/substrate#10279

Not tested, but including this change won't explicitly affect the clients until we remove the standalone node.