Feat/create billing plan

[MuthoniMN]

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Description

This PR adds an endpoint for creating a new billing plan.
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Related Issue (Link to Github issue)

#633
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Motivation and Context

The API doesn't have an endpoint to create a new billing plan. This is different from the source of truth(NestJS).

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How Has This Been Tested?

1. Run the app locally using php artisan serve
2. In Postman, send a POST request with the following:
   Headers:

{
   Authorization: "Bearer {your-token}"
}

Request Body:

{
    "name": "Discounted Plan",
    "duration": "Yearly",
    "is_active": false,
    "price": 20000,
    "description": "A discounted plan description"
}

3. You should get either of these responses:
   201 - Created Successfully

{
    "status": "success",
    "status_code": 201,
    "message": "Billing plan successfully created!",
    "data": {
        "name": "Discounted Plan",
        "duration": "Yearly",
        "price": 20000,
        "description": "A discounted plan description",
        "id": "9e52a242-b1ef-4395-ba8c-b6360be43c99",
        "updated_at": "2025-02-28T21:17:47.000000Z",
        "created_at": "2025-02-28T21:17:47.000000Z"
    }
}

403 - Duplicate Billing Plan

{
    "status": "error",
    "status_code": 403,
    "message": "Billing plan already exists!",
    "data": []
}

500 - Internal Server Error

{
    "status": "error",
    "status_code": 500,
    "message": "Internal Server Error!",
    "data": []
}

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Screenshots (if appropriate - Postman, etc):

Postman:

Passing Tests:

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Types of changes

• 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 change)

What this PR entails?

• Creation of a POST endpoint (api/v1/billing-plans)
• Updating the billing plan documentation
• Tests for the create billing plan method

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.