Add custom_attributes field to OrganizationMembership (#1458)

## Summary

Adds `custom_attributes` field to `OrganizationMembership` model to
expose custom attributes from identity providers.

## Changes

- Added `custom_attributes` field to OrganizationMembership type/model
- Field type: Record/Map/Dictionary of string keys to any values
- Always present, defaults to empty object `{}`
- Updated fixtures and tests

## API Field Details

The field will be present in:
- REST API responses
- Webhook events
- Events API responses

JSON field name: `custom_attributes` (snake_case)

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Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>