common : enable `--offline` mode without curl support (#16137)

common : enable `--offline` mode without curl support (#16137)

* common : use the json parser

Signed-off-by: Adrien Gallouët <redacted>
* common : enable --offline mode without CURL support

This change refactors the download logic to properly support offline mode
even when the project is built without CURL.

Without this commit, using `--offline` would give the following error:

    error: built without CURL, cannot download model from the internet

even if all the files are already cached.

Signed-off-by: Adrien Gallouët <redacted>
---------

Signed-off-by: Adrien Gallouët <redacted>