DirectivesHolder does not implement toString #4190

Describe the bug
#4027 introduced an unannounced breaking change in 25.0. Quite a few places that now contain a DirectivesHolder implicitly call toString on it in their own toString implementations. Before this change that worked just fine, because directives were just a list and lists tend to have reasonable toString implementations.

Here is one example:

DirectivesHolder, however, does not have one, so all those other toString implementations no longer produce reasonable output.

To Reproduce
Create a ScalarTypeDefinition object. Call toString on it.

Actual output:

ScalarTypeDefinition{name='Int', directives=graphql.language.NodeUtil$DirectivesHolder@5c995dee}

Expected output (and actual output in 24.x):

ScalarTypeDefinition{name='Int', directives=[]}