Labels, debug markers, and validation layers should be available on release builds

[aloucks]

Is your feature request related to a problem? Please describe.
I was trying to debug something in Renderdoc and none of the labels or debug groups were showing up. I switched to a debug build and my labels and groups started showing up. The issue is that debug builds can be painfully slow in some cases. I often just use release builds.

Describe the solution you'd like
I'd like to be able to set two flags during instance creation. One to enable debug markers, groups, and labels and a separate flag to enable validation. It looks like these already exist as flags on the hal Instance:

wgpu/wgpu-core/src/instance.rs

Lines 107 to 110 in 61fa8a3

	if cfg!(debug_assertions) {
	flags |= hal::InstanceFlags::VALIDATION;
	flags |= hal::InstanceFlags::DEBUG;
	}

Describe alternatives you've considered

1. I could enable debug_assertions in release builds or change debug builds to be more optimized. I'd still prefer a runtime flag during instance creation.
2. I could use the hal API to create the instance.

Additional context
The labels and debug groups have a cost associated with the string allocation but I think there's opportunity for improvement here by using string interning or other options to minimize the allocation.