Updates made to gossamer for Zombienet

## Task summary
This is a list of updates made to gossamer to enable Zombienet to work without changes:
- Added prometheus gauge `substrate_process_start_time_seconds`, this metric is used by zombienet to confirm the node has started. Not sure if we want metrics in gossamer that are in the substrate namespace, so consider different ways to handle this.
- Created flag `--rpc-cors` that zombienet uses when starting nodes, this has not been implemented in gossamer, just the flag has been added so that gossamer will not throw an error when it sees this flag.

## Other information and links

- See issue #2843 and PR #3098 and #3192