To better support printing of google.protobuf.Any messages (error details), this
also makes a few other changes:
1. Allows printing of unresolvable Any messages using an "@value" field in JSON output
that has the base64-encoded embedded message data.
2. Improves support for "-format text" to show expanded Any messages if possible.
(Due to limitations in underlying proto package, this will usually *not* be
that helpful. But this should greatly improve with v2 of the go protobuf API.)
3. Addresses a TODO in existing AnyResolver code to lazily fetch descriptors
as needed instead of having to download all files eagerly.
Sergei Vorobev