Better config parsing

[phijor]

No description provided.

[bmario]

@phijor Do you remember what's the current state of this?

[phijor]

Yes, the goals here were:

1. Modularize parsing of the configuration sent to the sink.
   In particular, objects are now initialized recursively from the configuration:

   class Foo:
       def __init__(self, bar: Bar):
           self.bar = bar
   
       @static_method
       def from_config(foo_config: dict) -> Foo:
           bar = some_processing(foo_config.get('bar', 'default'))
           returns Foo(bar=bar)
   
   class Quux:
       def __init__(self, foo: Foo):
           self.foo = foo
   
       @static_method
       def from_config(quux_config: dict) -> Quux:
           foo = some_more_processing(quux_config.get('foo', 42))
           returns Quux(foo=foo)

   This way, the constructors of Quux and Foo do not have to know about the particular format of the config, most importantly they don't care about how their fields get parsed from a constructors; they simply receive something that is already initialized.

2. Make classes testable. With 1. in place, Check in particular now becomes testable: Check.__init__ is now much more simple and does not care about what the configuration looks like, so you can simply supply some mock objects. On the other hand Check.from_config becomes testable. This for example immediately cought the bug in f968e24.

3. Have a uniform style of parsing the configuration at each level, in contrast to doing everything at once in Reporter._on_config().