use enum values and fix fields pydantic validation

[vincentsarago]

This PR does:

• fix validation issue when fields extension is enable for /items endpoint (return JSONResponse to avoid pydantic validation)
• use Relation enum value for POST Links (to match links for GET)
• avoid re-using post_search method to ease customization (mostly because we don't want to have to deal with a possible JSONReponse from the post_search method