Handle JSON downloads for TZIP-16 checks in LSP

Following #1994 (closed), we always pass json_dowload:None, and we should handle it.

These downloads need to be cached in other to not lag the language server, or some clever dealing with Lwt should be done to cancel previous downloads.

Acceptance criteria

• We display TZIP-16 checks that require downloads.
• We check that editing the contract won't lag the language server due to such downloads.
• Tests added.