Add toString() and equals() implementations for core domain types (Table, Row, Value, Column)

Hi,

for many relevant domain types like

- Table
- Row
- Value
- Column

there are currently no implementations of toString() and equals() (and, by implication, hashCode()). This is a significant blocker for making reliable assertions across different datasets and hinders test readability and diagnostics.

My current workaround is:

```
Assertions.assertThat(actualRows)
.usingElementComparator(new CustomRowComparator())
.withRepresentation(new CustomRowRepresentation())
.containsExactlyInAnyOrderElementsOf(expectedRows);
```

I would like to contribute a fix by adding consistent equals(), hashCode(), and toString() implementations for the classes above.