export SubscribeBook request model

[khancode]

High Level Overview of Change

Export SubscribeBook request model.

Context of Change

SubscribeBook can't be imported because it's not exported in xrpl/models/requests/__init__.py.

#394

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (non-breaking change that only restructures code)
• Tests (You added tests for code that already exists, or your new feature included in this PR)
• Documentation Updates
• Release

Test Plan

Verified unit tests still pass.