Improve Kotlin support

[marcphilipp]

Since Kotlin is the default language for Android apps and also has become increasingly popular for backend applications, we should resolve some long-standing and highly-voted issues to increase the usability when writing tests and extensions in Kotlin.

Deliverables

• Add Kotlin contracts to exposed Kotlin API #1866
  • Relax Kotlin contracts for Executable passed to assertTimeout #4551
• Support test methods with Kotlin suspend modifier #1914
• Add Kotlin Sequence support for @TestFactory, @MethodSource, and @FieldSource #3376
• Add JSpecify nullability annotations to Java APIs #4550