all: Support syncing extended attributes (fixes #2698) (#8513)

This adds support for syncing extended attributes on supported
filesystem on Linux, macOS, FreeBSD and NetBSD. Windows is currently
excluded because the APIs seem onerous and annoying and frankly the uses
cases seem few and far between. On Unixes this also covers ACLs as those
are stored as extended attributes.

Similar to ownership syncing this will optional & opt-in, which two
settings controlling the main behavior: one to "sync" xattrs (read &
write) and another one to "scan" xattrs (only read them so other devices
can "sync" them, but not apply any locally).

Co-authored-by: Tomasz Wilczyński <twilczynski@naver.com>

proto/lib/protocol/bep.proto

@@ -114,10 +114,15 @@ message FileInfo {
     // received (we make sure to zero it), nonetheless we need it on our
     // struct and to be able to serialize it to/from the database.
     uint32 local_flags = 1000;
+
     // The version_hash is an implementation detail and not part of the wire
     // format.
     bytes version_hash = 1001;
 
+    // The time when the inode was last changed (i.e., permissions, xattrs
+    // etc changed). This is host-local, not sent over the wire.
+    int64 inode_change_ns = 1002;
+
     bool deleted        = 6;
     bool invalid        = 7 [(ext.goname) = "RawInvalid"];
     bool no_permissions = 8;
@@ -151,6 +156,10 @@ message Counter {
 message PlatformData {
     UnixData    unix    = 1 [(gogoproto.nullable) = true];
     WindowsData windows = 2 [(gogoproto.nullable) = true];
+    XattrData   linux   = 3 [(gogoproto.nullable) = true];
+    XattrData   darwin  = 4 [(gogoproto.nullable) = true];
+    XattrData   freebsd = 5 [(gogoproto.nullable) = true, (ext.goname) = "FreeBSD"];
+    XattrData   netbsd  = 6 [(gogoproto.nullable) = true, (ext.goname) = "NetBSD"];
 }
 
 message UnixData {
@@ -171,6 +180,15 @@ message WindowsData {
     bool   owner_is_group = 2;
 }
 
+message XattrData {
+    repeated Xattr xattrs = 1;
+}
+
+message Xattr {
+    string name  = 1;
+    bytes  value = 2;
+}
+
 // Request
 
 message Request {