docs: added REST documentation for fields query string parameter to filter response fields

[IgnatG]

Closes #41

Summary

This PR add the relevant documentation for the fields query parameter in REST APIs, allowing consumers to specify which fields should be included in JSON responses.

Changes Made

API Guidelines Documentation

• Added new "Field Selection" section in pagination-filtering-sorting.md
• Usage examples including:
  • Basic field selection: GET /api/widgets?fields=id,name,createdAt
  • Combining with other parameters: GET /api/widgets?fields=id,name,status&status=active&limit=5
  • Default behaviour when the fields parameter is omitted
• Detailed behaviour specifications:
  • Valid fields - include only the requested fields in the response
  • Invalid field names - ignore silently and include only valid ones
  • Empty fields parameter - return all fields (same as omitting parameter)
  • Case sensitivity - field names are case-sensitive
  • Nested fields - support dot notation for nested objects (e.g., metadata.total)

OpenAPI Example Updates

• Added fieldsParam component in example.1.0.0.oas.yml
• Regex validation pattern to ensure proper field name format
• Multiple usage examples:
  • Basic fields: resultId,nhsNumber,result
  • Nested fields: resultId,nhsNumber,metadata.createdAt
  • Multiple fields: resultId,nhsNumber,type,result,metadata.createdAt,metadata.updatedAt
• Integrated with existing /results endpoint alongside pagination, filtering, and sorting parameters

[github-actions]

Test Results

36 tests   36 ✅  13s ⏱️
 4 suites   0 💤
 1 files     0 ❌

Results for commit e175b9c.

[andreyyudin]

Looks good to me

[github-actions]

🎉 Success! 🎉

PR successfully Fast Forward merged in ✅.