Idea: one more way to compare files when syncing with Google Drive (v3 api)

[spqr]

Idea: currently, there are two ways to compare files when dealing with Google Drive sync: by time and size and file content (byte-by-byte comparison)

However, I noticed that gdrive api v3 file metadata object exposes `md5Checksum` field.
Would be nice to add this as sync option. Previously for something like that to happen, you had to download file from a cloud (and there was not much sense to do it as byte-to-byte sync was doing pretty much same and being quicker)

With exposing of `md5checksum` property by gdrive for file object, we don't need to download a file anymore to calculate its checksum. The method would be slower than time and size comparison, but faster than byte-to-byte (esp. for big files/slower connections) as there is no need to parse file from a cloud, so hash needs to be calculated only on one side (which is local device in most cases), then compared with one in `md5checksum` field of gdrive object for given file. And it's also well balanced by a collision chance.