lib/api: Expose blocksHash in file info (#8810)

This adds the BlocksHash field from the FileInfo to our API output. It
can be useful for debugging, or for external tools. I'm intentionally
leaving it as an opaque base64 string because no meaning should be
derived from it: it's just a string.

lib/api/api.go

@@ -1828,6 +1828,7 @@ func fileIntfJSONMap(f protocol.FileIntf) map[string]interface{} {
 		"localFlags":    f.FileLocalFlags(),
 		"platform":      f.PlatformData(),
 		"inodeChange":   f.InodeChangeTime(),
+		"blocksHash":    f.FileBlocksHash(),
 	}
 	if f.HasPermissionBits() {
 		out["permissions"] = fmt.Sprintf("%#o", f.FilePermissions())