[Junit5 extension] add dedicated Annotations for Number, Boolean and String flags

[aepfli]

Problem

Our annotations for testing are excellent, and they greatly ease the developer experience. Currently, all the values need to be provided as a string. This is a limitation of java annotation as java annotation members can only be (https://docs.oracle.com/javase/specs/jls/se8/html/jls-9.html#jls-9.6.1):

• A primitive type
• String
• Class or an invocation of Class (§4.5)
• An enum type
• An annotation type
• An array type whose component type is one of the preceding types

Furthermore, annotations can't be generic, nor do they support inheritance.

@Flag(name = "BOOLEAN_FLAG", value = "true")

Goal

Evaluate the feasibility and effort of using meta-annotations or some kind of inheritance. We should not make the API more complex than it already is. Maybe we only support those convenience annotations for the simple annotation approach and not in the extended configuration.

like instead of

@Flag(name = "BOOLEAN_FLAG", value = "true")
@Flag(name = "STRING_FLAG", value = "true")
@Flag(name = "Number_FLAG", value = "1")

we would love to use

@BooleanFlag(name = "BOOLEAN_FLAG", value = true)
/* optional boolean improvement
@BooleanFlagTrue(name = "BOOLEAN_FLAG")
@BooleanFlagFalse(name = "BOOLEAN_FLAG")
*/
@StringFlag(name = "STRING_FLAG", value = "true")
@NumberFlag(name = "Number_FLAG", value = 1)