Typed config: move all Saluki-only default definitions to a single location in `defaults.rs`.

[webern]

Currently most of the Saluki-defaults are a bit obscurely defined in the Default implementation of the SalukiOnly struct. But in #2171 I instantiated what I think is a better pattern. Define the defaults in one reusable place, in the leaf crate, in a file named defaults.rs. The idea is that all system defaults can be found there.

This issue exists to ensure that we move all of them to there so that they are not hiding in impl Default for SalukiOnly or being stated in more than one place.

Note: this is not for Datadog schema defaults! That would violate the intent of #1802!