Add a new BEP field to be used by unresolved symlinks.

An unresolved symlink is a special kind of artifact in Blaze/Bazel that is tracked purely by the symlink target path (i.e., the result of readlink()) rather than the contents of the file it points to (which might not even exist, since unresolved symlinks are allowed to dangle). There's currently no way to represent them in an unambiguous way in the BEP.

PiperOrigin-RevId: 537286960
Change-Id: I1da1283513e937bcc75a11c29ddf65c424c922e6

src/main/java/com/google/devtools/build/lib/buildeventstream/proto/build_event_stream.proto

@@ -490,6 +490,8 @@ message File {
     string uri = 2;
     // The contents of the file, if they are guaranteed to be short.
     bytes contents = 3;
+    // The symlink target path, if the file is an unresolved symlink.
+    string symlink_target_path = 7;
   }
 
   // Digest of the file, using the build tool's configured digest algorithm,