feat(Index): Add overload for setSettings

[PLNech]

Following a doc feedback, I noticed all our docs examples for setSettings specify only one parameter. We could update all those doc examples, but to provide better DX in the spirit of DWIM, I propose we add an overload for this simple case.

I also took this opportunity to add a missing Nullable annotations to CompletionHandler.

[PLNech]

^: Test failing is still DNSTimeout. I'll relaunch the build, and merge if this is still the only one failing; I'll address this test following conversation with Bitrise in a later move.