Add Customer.subscriptions and mark CustomerSubscription.list_imported as deprecated

[wscourge]

• Added deprecation warning to Subscription.list_imported() method
• Added CustomerSubscription import to customer module
• Updated tests to use correct imports and assertions

[Copilot]

Pull request overview

This PR deprecates CustomerSubscription.list_imported() in favor of a new Customer.subscriptions() method and updates documentation examples to use correct field names.

• Adds new Customer.subscriptions() method for retrieving customer subscriptions
• Marks CustomerSubscription.list_imported() as deprecated with a warning
• Corrects field names from external_id to uuid in subscription connection examples

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
chartmogul/api/customer.py	Adds new Customer.subscriptions() method
chartmogul/api/customers/subscription.py	Implements deprecation warning for list_imported() method
test/api/test_customer.py	Adds test coverage for new subscriptions() method and updates field names in existing tests
README.md	Documents new subscriptions() method and corrects field names in examples

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