Send custom class attributes to Spyder without requiring the corresponding package to be installed

[CAM-Gerlach]

Problem Description

Initially discussed in spyder-ide/spyder#8852 .

The object explorer, either the one implemented in spyder-ide/spyder#8852 or the existing one based on the dictionary collections editor, does not work if the package the object's class is from is not installed, which is common under our recommended workflow of keeping Spyder in a separate environment from the working environment packages (especially important if their deps are incompatible with Spyder, or they could contaminate Spyder's environment with pip or conda-forge).

Therefore, we should provide a mechanism to at least read the object's attributes in this case. @ccordoba12 suggested:

I've been thinking about it and this is out of the scope of this PR. To solve it we need to create a pure text, json representation of an object in the kernel and send it to Spyder to display it in this new object editor. So, please open a new issue about it in spyder-kernels.

What steps reproduce the problem?

1. Open Spyder from a dedicated/base environment
2. Launch a kernel in a different environment with a package that provides custom classes
3. Instantiate an object of said custom class and attempt to view it in the Variable Explorer

What is the expected output? What do you see instead?

Expected: Being able to at least view the object in dict form.

Actual: An error message is produced, stating the package containing the class is not installed.