Provide a way to merge @SnapshotTestOptions

[skuzzle]

As the amount of configurable option grows, we must provide a way to merge hierarchically used options. Otherwise it would not be easily possible to refine the used options per test case.

@SnapshotTestOptions(attribute1 = nonDefaultValue, attribute2 = nonDefaultValue, attribute3 = nonDefaultValue)
class TestClass {

    @Nested
    @SnapshotTestOptions(attribute1 = nonDefaultValue)
    class InnerTestClass {
        @Test
        @SnapshotTestOptions(attribute1 = defaultValue, attribute2 = nonDefaultValue)
        void innerTestMethod(Snapshot snapshot) {
            // actual options:
            // attribute1 = defaultValue
            // attribute2 = nonDefaultValue
            // attribute3 = nonDefaultValue
        }
    }
}

Problem here is, that we need to distinguish between implicit and explicit default values.
attribute1 on the test method has an explicit default value which is intended to override a previously configured non-default value. Whereas attribute3 has an implicit default value which is not intended to override a previously configured non-default value

To be finoished:
Merging algorithm:

1. Traverse the hierarchy from bottom to top, starting at the testMethod
2. Read @SnapshotTestOptions from current element. If non is present, use SnapshoTestOptions.DEFAULT_OPTIONS
3. Read desired attribute from the obtained SnapshotTestOptions
4. If the attribute's value is not its default value, return it immediately
5. If the attribute's value is its default value, continue traversing the hierarchy until a non-default value is encountered

[skuzzle]

After giving this some more thought I think it is best to move away from annotation based config. There is no need for these settings be evaluated statically, so we could just as well move to a programmatic options API. That would be way more flexible and composable

[skuzzle]

Another idea to fix is is to use some kind of sentinel default value in the annotations that will be translated to the actual default value while evaluation the annotations. That would allow us to actually distinguish between real default values and default values that were explicitly specified by the user.