Improve backward compatibility tests for spec changes introduced in all table versions

[yyanyy]

Stem from this comment: https://github.com/apache/iceberg/pull/2521#issuecomment-828793923

We had a NPE bug (#2405) caused by reading a newly introduced field as boolean, which assigns null to this boolean field when reading old tables. While Iceberg has backward compatibility tests to test reading V1 tables, this field is introduced for all specs, and thus when creating V1 table for testing, this field will still be populated. NPE only occurs in a wider setting - during table commits (fixed in #2495) or reading manifest table (fixed in #2521).

I think to prevent this from happening in future, we may need to introduce a mechanism that creates table not through Iceberg code itself, so that the new fields introduced to the spec can continue to be null. The comment link above describes a proposal to achieve it but there might be better approaches.

[szehon-ho]

A bit late to the discussion, and didnt look very closely, but I wonder if we can make Testwriters for the various metadata that writes null for all optional fields, and then try to deserialize it with the regular readers, to test all newly-fields at once. @RussellSpitzer , @yyanyy

[szehon-ho]

Was chatting with @dramaticlly about it, cc if you want to look at it.

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