Is your feature request related to a problem? Please describe.
Currently ABI configuration supports only excludeClasses, excludeAnnotations.
When working with existing codebase, which does not follow package convention, it's hard to differentiate between public/internal api, bcz everything are essentially public, leading to dependency exposure if I follow plugin advices.

Describe the solution you'd like
Add to the abi { exclusions {} } possibility to configure:

• includeClasses('some.package')
• includeAnnotations('PublicApi')

In order to allow graceful migration and enforcements.

Describe alternatives you've considered

1. Major refactoring to use packages to differentiate api/internal package, will cause backward compatibility issues on the client and mess with git history. However, package separation is a common practice and used by java modules or ArchUnit.
2. As a workaround I came up with exclude everything except:

dependencyAnalysis {
    abi {
        exclusions {
            def includes = [
                'some.package.'
            ]
            def includesQuoted = includes.collect { Pattern.quote(it) }
            def excludeRegex = '^' + includesQuoted.collect { "(?!${it})" }.join('') + '.*$'

            ignoreGeneratedCode()
            excludeAnnotations('org.springframework.boot.autoconfigure.AutoConfiguration')
            excludeClasses(excludeRegex)
        }
    }
}

Additional context
If it makes sense I can contribute the change.
What I am not sure, how existing exclusion work with transitive class dependencies.
E.g. if we introduce inclusion configuration and include single class, I assume it will be challenging and non-explicit to add all dependent classes from class ABI.