API: 4 Collection Search Extension

[RobinGummels]

Implement the STAC Collection Search Extension with support for search parameters including full-text search (q), filtering, sorting, and pagination.

Acceptance Criteria:

• Search endpoint accepts query parameters (q, bbox, datetime, provider, license)
• Full-text search implemented across title, description, keywords
• Pagination working with limit and offset/token
• Sorting by configurable fields
• Response format compliant with Collection Search Extension

Reference: Section 6.1.2.3, Section 10.3 (AP-04, AP-07)