public void setSubstitutions(Map<String, Node> substitutions) {
        this.substitutions = Objects.requireNonNull(substitutions);
    }
Defines a map of strings to any JSON value to find and replace in the
generated OpenAPI model.

<p>String values are replaced if the string in its entirety matches one
of the keys provided in the {@code substitutions} map. The
corresponding value is then substituted for the string; this could
even result in a string changing into an object, array, etc.

@param substitutions Map of substitutions.
smithy-openapi/src/main/java/software/amazon/smithy/openapi/OpenApiConfig.java