[Ballerina to OAS] Refactor HATEOAS Support

[SachinAkash01]

Description:

private Map<String, Link> mapHateoasLinksToOpenApiLinks(String packageId, int serviceId,
                                                            FunctionDefinitionNode resourceFunction) {
        Optional<String> linkedTo = getResourceConfigAnnotation(resourceFunction)
                .flatMap(resourceConfig -> getValueForAnnotationFields(resourceConfig, BALLERINA_LINKEDTO_KEYWORD));

Rather than getting the string value of the node HateoasMapperImpl.java#L139, we can get as a
ExpressionNode value. Since IIRC syntax tree already parses the values and create them with proper structure. If we use that type, then we do not need to write our own parser logic like in here.. HateoasMapperImpl.java#L170

public static Optional<String> getValueForAnnotationFields(AnnotationNode resourceConfigAnnotation,
                                                               String fieldName) {
        return resourceConfigAnnotation
                .annotValue()
                .map(MappingConstructorExpressionNode::fields)
                .flatMap(fields ->
                        fields.stream()
                                .filter(fld -> fld instanceof SpecificFieldNode)
                                .map(fld -> (SpecificFieldNode) fld)
                                .filter(fld -> fieldName.equals(fld.fieldName().toString().trim()))
                                .findFirst()
                ).flatMap(SpecificFieldNode::valueExpr)
                .map(en -> en.toString().trim());
    }

In above code segment, instead of converting the expression node to a string, we can directly return the expression node to do this change. MapperCommonUtils.java#L481

Then in HateoasMapperImpl we need to visit the nodes accordingly in order to get the values of linkedTo field in the http:ResourceConfig annotation.