Add PDF::Bread /Metadata entry

Optional PDF 2.0 field.

lib/PDF/Bead-Thread.rakumod

@@ -39,10 +39,12 @@ role PDF::Thread does PDF::COS::Tie::Hash {
     use PDF::COS::Tie;
     use PDF::COS::Name;
     use PDF::Info;
+    use PDF::Metadata::XML;
 
     has PDF::COS::Name $.Type is entry where 'Thread';	# [name] (Optional) The type of PDF object that this dictionary describes; if present, is Thread for a thread dictionary.
     has PDF::Bead $.F is entry(:required, :alias<first>);	# [dictionary] (Required; is an indirect reference) The first bead in the thread.
     has PDF::Info $.I is entry(:alias<info>);	# [dictionary] (Optional) A thread information dictionary containing information about the thread, such as its title, author, and creation date. The contents of this dictionary shall conform to the syntax for the document information dictionary (see Link 14.3.3, “Document Information Dictionary” ).
+    has PDF::Metadata::XML $.Metadata is entry(:alias<metadata>); # [stream] (Optional; PDF 2.0; is an indirect reference) A metadata stream containing information about the thread, such as its title, author, and creation date (see 14.3.2, "Metadata streams")
 
 }