feat: Add offset-based pagination support

[sherlock2215]

Add Offset-Based Pagination Support

Problem

Currently, Kinto only supports token-based pagination which requires sequential access to records. Users cannot jump directly to specific record positions (e.g., records 100-120) without paging through all previous records.

Solution

This PR adds support for _offset parameter alongside the existing token-based pagination, allowing direct access to any record position.

Changes

• New _offset parameter: Allows specifying starting position for records
• Backward compatible: Existing token-based pagination continues to work unchanged
• Enhanced response format: Includes pagination metadata for offset requests
• Manual offset handling: Implements offset slicing for memory backend

Key Features

1. Direct Access: GET /records?_limit=20&_offset=100 returns records 100-119 directly
2. Pagination Metadata: Response includes pagination object with limit, offset, total, has_more
3. Next Page URLs: Automatically generated with correct offset values
4. Priority System: Offset takes precedence over token when both are provided

API Examples

# Offset pagination
GET /records?_limit=20&_offset=100

# Response
{
  "data": [...],
  "pagination": {
    "limit": 20,
    "offset": 100,
    "total": null,
    "has_more": true
  },
  "next": "/records?_limit=20&_offset=120"
}

# Backward compatibility maintained
GET /records?_limit=20&_token=abc123  # Still works

[sherlock2215]

This addresses the feature request in #3403

[leplatrem]

Oh wao! Thank you for taking the time to dive into the code-base and implement this new feature!
Great job, it's a valid idea :)

At this time of the year we are a bit short staffed and a bit busy to review it deeply. But we'll try our best, there are a lot of changes :)

From my first overlook, I can already tell you that before we can merge it, we must add documentation, unit and functional tests.

[leplatrem]

Do we want these .idea/ files? I'd say add it to .gitiignore

[sherlock2215]

sure ill addit:)

[leplatrem]

I'm not sure about this new pagination field. If we add it, we must check that it is consistent with everything else, and well documented. There is also the option to use response headers. Kinto responses haven't changed in 10 years 😅