Use execution_variable instead of context

This is mainly to be compatibe with other classes API.

In a lot of classes, we define methods like this:
```Eiffel
 method (req: WSF_REQUEST; res: WSF_RESPONSE)
    do
        ...
    end
```

With the context, the signature becomes:
```Eiffel
 method (ctx: C; req: WSF_REQUEST; res: WSF_RESPONSE)
    do
        ...
    end
```

So, I can't build a filter chain where one filter is with context
and one is without context (I can't call
WSF_FILTER.set_next (a_next: WSF_FILTER) with a filter that is a
descendant of WSF_CONTEXT_HANDLER).

Moreover, having to play with generic types just to pass some
data from one filter to another is a bit overkill imho.
Because this is really what I use contexts for:
to pass data from one filter to the next one.

Regarding execution_variable and strong typing, if we want to achieve these,
I realize we could write a class with one getter and one setter like this:

```Eiffel
  class
    TYPED_DATA

  feature -- Access

  user (req: WSF_REQUEST): detachable USER
    do
        if attached {USER} req.execution_variable ("user") as l_user then
            Result := l_user
        end
    end

  feature -- Element change

  set_user (req: WSF_REQUEST; a_user: USER)
    do
        req.set_execution_variable ("user", a_user)
    end
```

Now, I realize this is a major change since the last time we talked about this,
but at the end, after having played with both, I prefer the one with
execution_variable.