feat: delayed finalization (#214)

Adds `--finalize-delay-sec` cli argument, based on the great work by
@shunsukew at https://github.com/inkdevhub/swanky-node/pull/61.

Manual testing by starting node with/without option, along with example
contract upload:
```shell
# build example contract
cargo contract build --manifest-path=../ink-examples/erc20/Cargo.toml

# start node
cargo run

# upload contract
cargo contract upload --suri //Alice --execute --manifest-path=../ink-examples/erc20/Cargo.toml
# check finalized head remains at genesis
sleep 1
test $(curl -sH "Content-Type: application/json" -d '{"id":"1", "jsonrpc":"2.0", "method": "chainHead_unstable_genesisHash", "params":[]}' http://localhost:9944 | jq .result) \
  = $(curl -sH "Content-Type: application/json" -d '{"id":"1", "jsonrpc":"2.0", "method": "chain_getFinalizedHead", "params":[]}' http://localhost:9944 | jq .result) && echo PASS || echo FAIL


# start node (with delayed finalization)
cargo run -- --finalize-delay-sec 1

# upload contract
cargo contract upload --suri //Alice --execute --manifest-path=../ink-examples/erc20/Cargo.toml
# check finalized head matches chain head
sleep 1
test $(curl -sH "Content-Type: application/json" -d '{"id":"1", "jsonrpc":"2.0", "method": "chain_getHead", "params":[]}' http://localhost:9944 | jq .result) \
  = $(curl -sH "Content-Type: application/json" -d '{"id":"1", "jsonrpc":"2.0", "method": "chain_getFinalizedHead", "params":[]}' http://localhost:9944 | jq .result) && echo PASS || echo FAIL

```

Closes #160