Switch unixfs.Metadata.MimeType to optional

*** THIS IS A BREAKING CHANGE *** as per [1]: "Required is forever"

Nevertheless this seems like a good idea at this time: there are no known
producers ( nor consumers ) of MetaData nodes, and the current requirement
of MimeType has an extremely narrow application scope.

This change could very well be rejected in lieu of implementing a new type
of node ( e.g. TheRealMetadata ) in the DataType enum.

Based on ipfs#3451 (comment)

License: MIT
Signed-off-by: Mib Kd743naq <[email protected]>

[1] https://developers.google.com/protocol-buffers/docs/proto#specifying-field-rules

Author: mib-kd743naq

unixfs/pb/unixfs.pb.go

@@ -16,5 +16,5 @@ message Data {
 }
 
 message Metadata {
-	required string MimeType = 1;
+	optional string MimeType = 1;
 }