Add support for _source_code_ attribute for runtime generated class definitions

[leonardt]

For fault, I am trying to do a runtime generated class definition (need to do rewrites to add logic that converts the function interface from magma types to system verilog types, so we need to do things like flatten the arguments for tuples). inspect.getsource doesn't work when we use exec to define a class (since it tries to look for the file it was defined in. This adds a special magic attribute that a downstream tool can set on a class that provides the source code so we can avoid using inspect.

[leonardt]

Here's an example use case: https://github.com/leonardt/fault/pull/299/files#diff-bab5267afbc634ff9f75f9ad1e6b92f205927079d9e4d2f9da81c64039942dc5R95

[leonardt]

Here's the error when it tries to find the source: https://github.com/leonardt/fault/runs/1883703264#step:6:717