Add Features::MULTI_DRAW_INDIRECT to Metal

[expenses]

Description

We currently emulate MDI in Metal on top of draw_primitives_indirect:

wgpu/wgpu-hal/src/metal/command.rs

Lines 847 to 850 in a3b2418

	for _ in 0..draw_count {
	encoder.draw_primitives_indirect(self.state.raw_primitive_type, &buffer.raw, offset);
	offset += mem::size_of::<wgt::DrawIndirectArgs>() as wgt::BufferAddress;
	}

Therefore we might as well expose the feature as long as it's clear that it's emulated and not natively supported.

Testing

I don't have my Macbook on me at the moment but I can test it on that when I get home.