Add support for nullable primitive lists

[skarimo]

Motivation
Previously, the sdk did not support nullable arrays. With introduction of new nullable properties and promotion of old properties to nullable, we now have use cases where we need to differentiate between explicit null vs zero value.

Changes in the PR
This PR introduces a new wrapper around nullable primitive lists NullableList[T any]. This is a backward incompatible change but one that is necessary to add support for explicit null lists.

A previously nullable list with type []string will now be converted to datadog.NullableList[string]. There is a helper function NewNullableList. As an example:

RestrictedRoles: []string{
	"1234-567-8910",
}

Would now be configured as:

RestrictedRoles: *datadog.NewNullableList(&[]string{
	"1234-567-8910",
})

[therve]

This looks good to me, but it's a breaking change so it should be marked as such. Can you also edit the PR description to explain the change a bit and what do to? This is our best way to document a migration, as it's linked in the changelog.