Change IMapper visibility from private to protected

[gpcaretti]

In order to simply the extension of the AutoMapperObjectMapper, this proposal want to promote the IMapper attribute to protected.
This allows to extend the IObjectMapper without realizing a new version of Abp.

See #4882 and #4885

[ryancyq]

@gpcaretti not sure how useful it would be to make mapper a protected property.

I would imagine if you want to extend default abp object mapper, you would need to create custom object mapper class (and extend from abp default implementation) and replace it.

Wouldn’t it the same if the developer creates a custom object mapper without extending from about default implementation ?

[ryancyq]

Nonetheless, if it is useful for developers, I think we still can lift the restriction here.

Please rename _mapper to Mapper as we use PascalCase for protected property

[gpcaretti]

Done

[ryancyq]

You will need to rename usage of _mapper as well.

[gpcaretti]

@ryancyq said:

Wouldn’t it the same if the developer creates a custom object mapper without extending from about default implementation ?

This is reasonable, as currently the class is very simple. By the way, inheritance has its advantages of avoiding rewriting basic methods present in the ancestor.

Evaluate the code and feel free to (not) adopt it. Just let me know your decision, in order to close the PR.