Add list, copy and inject variables values

[echarles]

The client should be able to list the variables, with their types and size in bytes (ideally), via a list_vars() method.

It should also be able to copy the value on demand via a get(var_name, mime_type = "text/plain") method.

Question: should the API be generic for all kernel languages? What about starting with only Python kernels?

Similarly it should be able to inject the content of a variable via a set(var_name, value) method.