plugin sec aesgcm: new plugin handling `aesgcm:` scheme for e2e encrypted media sharing: The scheme is register with download meta plugin, and it is activated with HTTP Upload (XEP-0363) when `encryption` is used in `options` with the value of C.ENC_AES_GCM. This is also known as `OMEMO Media Sharing` even if this method is not directly linked to OMEMO and can be used independently.