Default to not allowing write operations, allow for a way to override this so write operations can be specified

[echarrod]

Description

Currently, the mcp-luno server exposes all tools including those that perform write operations (e.g., create_order, cancel_order) as soon as API credentials are provided. We should add an additional security layer by requiring an explicit opt-in environment variable to enable write operations.

Requirements

1. Add an environment variable ALLOW_WRITE_OPERATIONS that controls whether write operations are exposed
2. Default behavior should be read-only (no write operations available)
3. Write operations include:

• create_order
• cancel_order

Implementation Details

1. Add configuration option in config.Config to track this setting
2. Modify tool registration to check this flag before exposing write operation tools
3. Update server initialization to parse this environment variable
4. Update documentation in README.md to explain this feature