protobuf: Support custom ProtobufCodec implementations

Some object types might not be annotated with @Column, but we may
still want to embed them into a protobuf format.  Treat these as
opaque binary messages using the length delimited message format,
encoding them like any other nested message or arbitrary binary.

Change-Id: I8139e36838cbb2eabacbf3eab8c0156ddbdeb29b
Signed-off-by: Shawn O. Pearce <sop@google.com>
6 files changed