Make DefaultContentManager as override able

[hyzx86]

I wanted to rewrite The DeaultContentManager, but there are some problems ：

1. Data is a property of the Internal modifier that prevents me from accessing this property in other assemblies
2. Define all the methods in DefaultContentManager as virtual methods

[hishamco]

What sort of modifications you may change in the default implementation?

[ns8482e]

@hyzx86 why you would rewrite content manager?

[ns8482e]

There are other framework features available for CRUD of entity other than content item

[hyzx86]

Here I have create a PR ，but it still at there many days
#11532

Here I created a PR, but it's still there for many days
I'm going to rewrite the bulk import function,
And some error event interception
OC's PR audit was so slow that I could only quickly implement these changes by modifying the code.
However, these Settings prevented me from overwriting some features

[hyzx86]

In this Issue it is mentioned to change the way the Handler executes, such as session.CancelAsync() when the Handler reports an error.
#11643

[hyzx86]

It seems to have been solved by internal invocation😂