added basic TLV impl

src/cfdp/pdu/metadata.rs

@@ -0,0 +1,15 @@
+use crate::cfdp::pdu::{FileDirectiveType, PduHeader};
+use crate::cfdp::ChecksumType;
+
+pub struct MetadataParams {
+    closure_requested: bool,
+    checksum_type: ChecksumType,
+    file_size: u64,
+    //src_file_name:
+}
+
+pub struct MetadataPdu {
+    pdu_header: PduHeader,
+    file_directive: FileDirectiveType,
+    metadata_params: MetadataParams,
+}