refactor: make PartitionSpec ctor private and add factory methods with validation

[HeartLinked]

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[wgtmac]

Suggested change

	spec, PartitionSpec::Make(*schema, spec_id, std::move(partition_fields),
	true /*allow_missing_fields*/));
	spec, PartitionSpec::Make(*schema, spec_id, std::move(partition_fields),
	/*allow_missing_fields=*/false));

It is illegal for the current spec to have a missing source field. You can verify it by checking the Java code below that bind(schema) is called.

  public PartitionSpec bind(Schema schema) {
    return copyToBuilder(schema).build();
  }

  public PartitionSpec bind(Schema schema, boolean ignoreMissingFields) {
    return copyToBuilder(schema).build(ignoreMissingFields);
  }