Make Executor session handling safer

[MoritzWillmann]

Problem

The current handling of Qiskit IBM Runtime Sessions in the Executor class is extremely unsafe. In most cases the user will have to manually close the session after use or else it could use valuable computation time. In my initial conception this mostly happens in two cases (could be more):

1. Something goes wrong after the Executor creates the Session and the destructor (which was supposed to close the session) is never called
2. The Executor is kept in the context long time after usage (e.g. in the scope of a notebook) and thus the Session is never closed

It could also be related to the Executor not being released properly after deletion (which could also be related to #358), but this is just a hunch and would need further investigation.

Approach

My approach in this PR to mitigate these problems:

• Encourage context managed use such as

with Executor(...) as executor:
    ...

• Use weakref.finalizer instead of __del__, since it's apparantly safer and always assign when Session is used for cases when context managed usage is not feasible or users forget to use

Still Open Points

• fix tests, it seems the finalizer is not called in some of the tests
• adapt documentation to encourage context managed approach
• create warning when not using context managed approach
• cleanup to make more robust and conform to styling