Diff similar DataFrames with ignoreColumns

[EnricoMi]

With ignore columns, diffing allows for different Dataset types.

Fixes #108.

[github-actions]

Test Results

   106 files  ±  0     106 suites  ±0   21m 24s ⏱️ +53s
   340 tests +  7     340 ✔️ +  7  0 💤 ±0  0 ❌ ±0 
3 264 runs  +70  3 264 ✔️ +70  0 💤 ±0  0 ❌ ±0 

Results for commit 853ea7e. ± Comparison against base commit bceea91.

This pull request removes 2 and adds 9 tests. Note that renamed tests count towards both.

uk.co.gresearch.spark.diff.DiffSuite ‑ diff with different number of column
uk.co.gresearch.spark.diff.DiffSuite ‑ diffWith ignored

uk.co.gresearch.spark.diff.DiffSuite ‑ diff similar with ignored column and different number of columns
uk.co.gresearch.spark.diff.DiffSuite ‑ diff similar with ignored columns
uk.co.gresearch.spark.diff.DiffSuite ‑ diff similar with ignored columns and empty schema
uk.co.gresearch.spark.diff.DiffSuite ‑ diff similar with ignored columns case-insensitive
uk.co.gresearch.spark.diff.DiffSuite ‑ diff similar with ignored columns case-sensitive
uk.co.gresearch.spark.diff.DiffSuite ‑ diff similar with ignored columns of different type
uk.co.gresearch.spark.diff.DiffSuite ‑ diff with different number of columns
uk.co.gresearch.spark.diff.DiffSuite ‑ diff with ignored columns of different types
uk.co.gresearch.spark.diff.DiffSuite ‑ diff with no id columns ids taken from left

♻️ This comment has been updated with latest results.

[EnricoMi]

@btt3165 ignore columns allow to diff "similar" Dataset[T] and Dataset[U], not just identical Dataset[T] and Dataset[T].

What do you think of this generalization?