Add connection pre-warming for DynamoDB async client #6060

# Current implementation - client created but no connections established
async def initialize(self, config: RepoConfig):
    online_config = config.online_store
    await self._get_aiodynamodb_client(...)  # Creates client object only

import time
from feast import FeatureStore

fs = FeatureStore(repo_path=".")

# First request - includes connection setup overhead
start = time.time()
fs.get_online_features(features=["fv:f1"], entity_rows=[{"id": "1"}])
first_request = time.time() - start  # ~80ms

# Second request - connections already warm
start = time.time()
fs.get_online_features(features=["fv:f1"], entity_rows=[{"id": "1"}])
second_request = time.time() - start  # ~60ms

print(f"First: {first_request*1000:.0f}ms, Second: {second_request*1000:.0f}ms")
# Output: First: 80ms, Second: 60ms (20ms cold-start overhead)

async def initialize(self, config: RepoConfig):
    online_config = config.online_store
    
    client = await self._get_aiodynamodb_client(
        online_config.region,
        online_config.max_pool_connections,
        # ... other params
    )
    
    # Warm up connection pool with a lightweight call
    if online_config.warmup_connections:  # New config option
        try:
            await client.describe_limits()
        except Exception:
            pass  # Ignore errors, just warming up connections