Feat: Normalize TableMetadata

[c-thiel]

This is a part of the TableMetadataBuilder (#587).

Normalization of TableMetadata does:

• Validate the current schema is set and valid
• Validate that all refs are valid (snapshot exists)
• Validate logs are chronological
• Normalize location (remove trailing slash)
• Validate that for V1 Metadata the last_sequence_number is 0
• If the default partition spec is specified but the spec is not present in specs, add it
• If the default sort order is unsorted but the sort order is not present, add it

In Java most of the validations can be found in the constructor:
https://github.com/apache/iceberg/blob/ab2c6f889d07eeee51a1f58605be248e9330d91b/core/src/main/java/org/apache/iceberg/TableMetadata.java#L265

[Xuanwo]

Thank you very much for working on this. I have a few style suggestions, but please feel free to disregard them if they don't suit you.

[Xuanwo]

Those comments are nice. Can we improve them by moving to related functions? It's possible for us to refactor or change parts of them, which may make the comments easier to be invalid.

[Xuanwo]

Hi, I'm not sure if this makes the logic easier to read. (It works for me).

if self.partition_spec_by_id(self.default_spec_id).is_some() {
   return Ok(())
}

if self.default_spec_id != DEFAULT_PARTITION_SPEC_ID {
  return Err(...)
}

let partition_spec = xxx;
self.partition_specs.insert(xxx);
Ok(())

The self.partition_specs.is_empty() check can be avoided since it should never occur.

[Xuanwo]

I have the same question.

[c-thiel]

Thank you very much for working on this. I have a few style suggestions, but please feel free to disregard them if they don't suit you.

Thanks @Xuanwo, they all make sense! Addressed in Improve readability & comments

[Xuanwo]

Thank you for your work, nice comments and great tests! I love them.