[core] PassChannel is not expresive enough to match spec

[sagudev]

Is your feature request related to a problem? Please describe.
Spec says that you either specify readonly: true or pass ops:

If format has a depth aspect and this.depthReadOnly is false:
this.depthLoadOp must be provided.
this.depthStoreOp must be provided.
Otherwise:
this.depthLoadOp must not be provided.
this.depthStoreOp must not be provided.

and wgpu-core currently does not support not passing ops.

Describe the solution you'd like
Something that would match specs better:

pub struct PassChannel<V> {
    pub load_op: Option<LoadOp>,
    pub store_op: Option<StoreOp>,
    pub clear_value: V,
    pub read_only: bool,
}

Describe alternatives you've considered
We could use more high level solution, but that would be harder to use in browser impl (not expressive enough for invalid cases, so something like this would make more sense in wgpu):

enum Chan {
    ReadOnly,
    Ops {load_op: LoadOp, store_op: StoreOp}
}

Additional context
Relevant CTS tests: https://gpuweb.github.io/cts/standalone/?q=webgpu:api,validation,render_pass,render_pass_descriptor:depth_stencil_attachment,loadOp_storeOp_match_depthReadOnly_stencilReadOnly:*

[sagudev]

We need to do validation as part of pass creation (beginRenderPass), more specifically in fill_arc_desc, so my plan is to for Arc to have resolved Arc channel (readonly + ops) while unarced (public) desc will have new interface to match the spec.

[teoxoy]

Sounds good to me.

Related: #2944