feat: add configurable collection name via adapterOptions

[Copilot]

The adapter hardcoded casbin_rule as the MongoDB collection name, preventing users from following their own database naming conventions.

Changes

• MongooseAdapterOptions interface: Added optional collectionName parameter
• Schema function: Accepts custom collection name, falls back to default
• Adapter constructor: Passes custom collection name to both schema and model creation
• Unit tests: Verify default behavior and custom collection name functionality
• Documentation: Added usage example and configuration section

Usage

const adapter = await MongooseAdapter.newAdapter(
  'mongodb://localhost:27017',
  {},
  { collectionName: 'casbinRule' }  // Use camelCase or any naming convention
);

When collectionName is omitted, the adapter uses casbin_rule as before.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Possibility to set collection name for the adapter</issue_title>
<issue_description>It would be great to be able to set collection name as an option in adapterOptions

Currently adapter is using hardcoded casbin_rule name

We'd like to use camelCase to be in line with our db naming convention
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Possibility to set collection name for the adapter #75

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[hsluoyz]

@nodece @Shivansh-yadav13 please review

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