feat(scripts): mainnet.sh (celestiaorg#3166)

Totally optional PR to add a `mainnet.sh` script that can be used to
start a consensus full node on mainnet. I'm using this to try to sync
from scratch with a celestia-appd binary built from `main`.

Example script output

```
root@rootul-mainnet-consensus:~/celestia-app# ./scripts/mainnet.sh
celestia-app home: /root/.celestia-app
celestia-app version: 1.0.0-rc0-564-g82703272

Are you sure you want to delete: /root/.celestia-app? [y/n] y
Deleting /root/.celestia-app...
Initializing config files...
Settings seeds in config.toml...
Downloading genesis file...
Starting celestia-appd in the background and piping logs to mainnet.log
You can check the node's status via: celestia-appd status
```

scripts/mainnet.sh

@@ -0,0 +1,44 @@
+#!/bin/sh
+
+# Stop script execution if an error is encountered
+set -o errexit
+# Stop script execution if an undefined variable is used
+set -o nounset
+
+CHAIN_ID="celestia"
+NODE_NAME="node-name"
+SEEDS="e6116822e1a5e283d8a85d3ec38f4d232274eaf3@consensus-full-seed-1.celestia-bootstrap.net:26656,cf7ac8b19ff56a9d47c75551bd4864883d1e24b5@consensus-full-seed-2.celestia-bootstrap.net:26656"
+CELESTIA_APP_HOME="${HOME}/.celestia-app"
+CELESTIA_APP_VERSION=$(celestia-appd version 2>&1)
+
+echo "celestia-app home: ${CELESTIA_APP_HOME}"
+echo "celestia-app version: ${CELESTIA_APP_VERSION}"
+echo ""
+
+# Ask the user for confirmation before deleting the existing celestia-app home
+# directory.
+read -p "Are you sure you want to delete: $CELESTIA_APP_HOME? [y/n] " response
+
+# Check the user's response
+if [ "$response" != "y" ]; then
+    # Exit if the user did not respond with "y"
+    echo "You must delete $CELESTIA_APP_HOME to continue."
+    exit 1
+fi
+
+echo "Deleting $CELESTIA_APP_HOME..."
+rm -r "$CELESTIA_APP_HOME"
+
+echo "Initializing config files..."
+celestia-appd init ${NODE_NAME} --chain-id ${CHAIN_ID} > /dev/null 2>&1 # Hide output to reduce terminal noise
+
+echo "Settings seeds in config.toml..."
+sed -i.bak -e "s/^seeds *=.*/seeds = \"$SEEDS\"/" $CELESTIA_APP_HOME/config/config.toml
+
+echo "Downloading genesis file..."
+celestia-appd download-genesis ${CHAIN_ID} > /dev/null 2>&1 # Hide output to reduce terminal noise
+
+echo "Starting celestia-appd in the background and piping logs to mainnet.log"
+nohup celestia-appd start > "${HOME}/mainnet.log" 2>&1 &
+
+echo "You can check the node's status via: celestia-appd status"