[Enhancement] Add more complete CRUD to create MCP Gateway and Gateway Targets

While it is super easy to do this:

```python
from bedrock_agentcore_starter_toolkit.operations.gateway.client import GatewayClient

client = GatewayClient(region_name=region_name)
gateway = client.create_mcp_gateway()
```

There is no way to update or delete the MCP Gateway. Please provide two new methods to the `GatewayClient` class:

1. `update_mcp_gateway()`: change any config of the MCP Gateway
2. `delete_mcp_gateway()`: delete all the resources created with the `create_mcp_gateway()` API

Same thing for gateway targets.

Start by reading the GatewayClient class and its create_mcp_gateway() implementation to understand which gateway and target resources it creates. Define the update and delete behavior for gateways and gateway targets, including cleanup of all resources created by the API, then verify that the complete CRUD operations work.

Written by the indexing model from the issue text.