refactor(fuel-core-nats): improve fuel core nats dev script

This change allows creating a .env file matching the templated environment variables from
.env.sample to start the fuel-core-nats service locally.

.env.sample

@@ -1,2 +1,6 @@
-INFURA_API_KEY=2d0fcc5e22...
-GENERATED_P2P_SECRET=930ge1d26cdd143e401923e69ca5d...
+KEYPAIR=generated-p2p-secret
+RELAYER=https://sepolia.infura.io/v3/infura-api-key
+RELAYER_V2_LISTENING_CONTRACTS=0x768f9459E3339A1F7d59CcF24C80Eb4A711a01FB
+RELAYER_DA_DEPLOY_HEIGHT=5791365
+RELAYER_LOG_PAGE_SIZE=2000
+SYNC_HEADER_BATCH_SIZE=100

Makefile

@@ -27,9 +27,9 @@ setup: check-commands
 # ------------------------------------------------------------
 #  Development
 # ------------------------------------------------------------
-
-dev/run-node:
-	./scripts/run-node.sh
+# Starts fuel-core-nats service
+start.fuel-core-nats:
+	./scripts/start-fuel-core-nats.sh
 
 dev-watch:
 	cargo watch -- cargo run

scripts/start-fuel-core-nats.sh

@@ -0,0 +1,49 @@
+#!/bin/bash
+
+# Load environment variables from .env file
+if [ -f .env ]; then
+    export $(grep -v '^#' .env | xargs)
+else
+    echo ".env file not found. Please create a .env file using the '.env.sample' template and try again."
+    exit 1
+fi
+
+# Function to check if environment variables are set
+check_env_vars() {
+    local missing_vars=0
+
+    while IFS= read -r line; do
+        # Skip empty lines and comments
+        [[ -z "$line" || "$line" == \#* ]] && continue
+
+        # Extract the key
+        key=$(echo "$line" | cut -d '=' -f 1)
+
+        # Check if the key is set in the environment
+        if [ -z "${!key}" ]; then
+            echo "Environment variable $key is not set."
+            missing_vars=$((missing_vars + 1))
+        fi
+    done < ".env.sample"
+
+    return $missing_vars
+}
+
+check_env_vars
+
+cargo run -p fuel-core-nats -- \
+    --service-name "NATS Publisher Node" \
+    --ip 0.0.0.0 \
+    --port 4000 \
+    --peering-port 30333 \
+    --db-path tmp/fuel-core-db  \
+    --utxo-validation \
+    --poa-instant false \
+    --enable-p2p \
+    --keypair $KEYPAIR \
+    --sync-header-batch-size $SYNC_HEADER_BATCH_SIZE \
+    --enable-relayer \
+    --relayer $RELAYER \
+    --relayer-v2-listening-contracts $RELAYER_V2_LISTENING_CONTRACTS \
+    --relayer-da-deploy-height $RELAYER_DA_DEPLOY_HEIGHT \
+    --relayer-log-page-size $RELAYER_LOG_PAGE_SIZE \