Added method to get waiting pipelines IDs from pipeline cache. (#12874)

# Objective
- Add a way to easily get currently waiting pipelines IDs.

## Solution
- Added a method to get waiting pipelines `CachedPipelineId`.

---------

Co-authored-by: James Liu <contact@jamessliu.com>

crates/bevy_render/src/render_resource/pipeline_cache.rs

@@ -494,10 +494,16 @@ pub struct PipelineCache {
 }
 
 impl PipelineCache {
+    /// Returns an iterator over the pipelines in the pipeline cache.
     pub fn pipelines(&self) -> impl Iterator<Item = &CachedPipeline> {
         self.pipelines.iter()
     }
 
+    /// Returns a iterator of the IDs of all currently waiting pipelines.
+    pub fn waiting_pipelines(&self) -> impl Iterator<Item = CachedPipelineId> + '_ {
+        self.waiting_pipelines.iter().copied()
+    }
+
     /// Create a new pipeline cache associated with the given render device.
     pub fn new(
         device: RenderDevice,