Introduce AotTestAttributes mechanism in the TestContext framework

[sbrannen]

Overview

For certain use cases it would be beneficial to be able to compute something during AOT build-time processing and then retrieve the result of that computation during AOT run-time execution.

Use Case

The SpringBootTestContextBootstrapper performs class path scanning for the @SpringBootConfiguration class using a utility called AnnotatedClassFinder; however, class path scanning does not work within a GraalVM native image.

In the Spring Native project, the AnnotatedClassFinder worked without modification because Spring's component indexer was used to index annotated components during the build; however, in Spring Framework 6.0 and Spring Boot 3.0 we do not want to rely on the component indexer.

Ideally, SpringBootTestContextBootstrapper should be able to:

• Use the AnnotatedClassFinder during AOT build-time processing.
• Store the class name of the @SpringBootConfiguration class for later retrieval.
• Retrieve the @SpringBootConfiguration class name that was discovered at build time during AOT run-time execution.

Proposal

Introduce an AotTestAttributes mechanism in the Spring TestContext Framework.

• conceptually similar to org.springframework.core.AttributeAccessor
• must allow an AOT-aware component to contribute a key-value pair, where the key is a String and the value is an String
• initial support should provide convenience methods for retrieving an attribute as a String or boolean
  • if deemed necessary, generic support can later be added for providing a CodeBlock or statement to create the object in the attributes store
• must generate the necessary source code during the AOT processing phase in the testing framework to create a persistent map of the attributes
• must provide a mechanism for accessing the stored attributes during AOT run-time execution

[sbrannen]

I've pushed a working prototype for TestAotProperties here:

main...sbrannen:TestAotProperties

@philwebb, let's see if this meets the needs of SpringBootTestContextBootstrapper.