config InitDefaults

go-ucfg's [`Unpack`](https://pkg.go.dev/github.com/elastic/go-ucfg#Config.Unpack) method calls `InitDefaults` on the passed item along with any of it's children that implement the function.

However we explicity call the children's `InitDefaults` from the parent:
https://github.com/elastic/fleet-server/blob/0718a55b3152a902f3c47bb841afa78cb166231b/internal/pkg/config/input.go#L155-L160
and this continues for a few extra layers:
https://github.com/elastic/fleet-server/blob/0718a55b3152a902f3c47bb841afa78cb166231b/internal/pkg/config/input.go#L96-L109

These extra `InitDefaults` calls are not needed; the input's `InitDefaults` can probably be reduced to
```go
func (c *Input) InitDefaults() {
c.Type = fleetInputType
}
```
and most other structs in our config package can be redone as well.
We should try to clean this up to reduce extra calls.