This discussion is a follow up of this comment, in summary, I'm thinking about make PartitionSpec safe with following changes:

1. Making all fields private to this struct.
2. Add a schema field to PartitionSpec.

This will introduce several changes:

1. PartitionSpec could only be built using builder, which has checks for serveral parts to ensure it's correct
2. PartitionSpec's partition type could be infered by itself
3. We may need to change the deserialization of TableMetadata to use builder to build it.