Convert DefaultTableMapping::setFieldNames() and ::setExtraColumns() to protected methods

Just stumbled upon this. Currently there is not really an easy way to modify the default table mapping, in my opinion. Unless you actually want something radically different providing a different (sub-)class other than DefaultTableMapping, while possible, is not actually very helpful, because you may need to copy the entire class just to change a few lines. Having these methods public, currently allows avoiding that and actually making small alterations with just a small amount of code. See 🐛 Cannot use UUID as entity ID Needs work for something that is currently fixable fairly cleanly from custom code which would become impossible, not in principle, but in reality, if these methods were made protected.

Since this also does not bring any maintainability gain as far as I can tell, I am against this until we have a different more bespoke API for actually customizing the entity table layout.

The methods setFieldNames() and setExtraColumns() are called inside the class only. So, it can be converted into the protected.

There is another conversion of renaming setFieldNames the function on issue 📌 Replace DefaultTableMapping::setFieldNames() with ::add*() and ::remove*() Needs work . If the approach is taken place we have to reroll this patch by making a protected method.

Re #16: As I noted in #12 these methods are helpful for contrib/custom code, so I don't think your argument is very convincing

For more discussion.

Also don't believe we can change this in a minor so if we still decide to update we can't do until D12