op-proposer: support stopped flag like batcher

## What

Support `stopped` flag on `op-proposer` like `op-batcher`.

`stopped` flag is a optional command like flag and it makes the application disabled state when the application started.

## Why

Would be useful if the standby op-proposer required.

## How
- Adding `stopped` flag on CLI side
- Check the flag when op-propser is started.