Add ResultAssert

[sviperll]

Replace #8 to merge into non-main branch

[marcindabrowski]

Hey @sviperll, any progress on this?
Do you want me to work on this?

[sviperll]

I'm actually quite busy lately. I'd like to work on this tomorrow. One thing that you may want to work on is to

1. Add documentation (javadoc) to ResultAssert class, it's good to add a couple of examples there and probably add some documentation to some methods.

2. I'm not sure if ResultAssert#hasErrorThat(Consumer) is an idiomatic assertj-API. Are there similar methods that accept Consumer as an argument? I haven't seen such methods, I would prefer to avoid it completely or to research some alternatives. From my own quick research, I'm thinking that maybe better alternative is to return AbstractAssert (For Object-type, but this is ok, since we can do asInstanceOf to specialize). Your example would become:

        ResultAssert.assertThat(result)
                   .isError()
                   .hasErrorThat()
                   .isInstanceOf(NumberFormatException.class);