Support default parameters for DgsData methods

[kilink]

Add support for Kotlin default parameter values for DgsData methods.

This is a follow up to PR #1302, where we added support for Kotlin default parameters when mapping input arguments; with this PR we now also support default parameters for the DgsData methods themselves.

Example:

@DgsComponent
class DataFetcher {
    @DgsData(parentType = "Query", field = "hello")
    fun fetcher(@InputArgument input: List<String> = emptyList()): String {
        // ...
    }
}

If the schema does not require input and the query did not supply it, previously this would fail, even though the fetcher method is written in a way to handle the missing input. This PR updates the DataFetcherInvoker to call Kotlin methods reflectively using callBy which allows supporting default parameters by omitting values in the passed in Map when the parameter is optional.

[srinivasankavitha]

Could you explain what lines #82 and #83 are doing?

[kilink]

We are checking if the KFunction takes an instance parameter, and supplying it if so; that is, the target object we want to call the method on, in this case the DGS component. That said, I can't really think if a scenario where we won't have an instance parameter, but we are kind of forced to deal with the possibility.

[srinivasankavitha]

Thanks for the PR! Looks good to me. fyi - we now have 2 branches we merge PRs to, the master is on Spring Boot 3.0 and we have a spring-boot-2.7 branch as well.