Add basic strapi5 compatibility option

[DeppsOpz]

This PR adds a strapi5 compatibility option to address some breaking changes made in the API. This has been tested with a fairly basic strapi5 setup, so not all breaking changes in v5 may be covered.

• Added property v5compat as site option
• Add HTTP header Strapi-Response-Format: v4 to requests if v5compat is true (https://docs.strapi.io/dev-docs/migration/v4-to-v5/breaking-changes/new-response-format)
• Use document.documentId instead of document.id when making requests if v5compat is true (https://docs.strapi.io/dev-docs/migration/v4-to-v5/breaking-changes/use-document-id)
• Added information to README.md
• Bumped version