processors: introduce input queues

Frame processors can now decide if they should continue processing frames or
not, and if so also decide when to continue processing frames. For example,
asynchronous TTS services will stop processing frames until they have generated
all the audio for an LLM response.

CHANGELOG.md

@@ -9,6 +9,17 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- There's now an input queue for each frame processor. When you call
+  `FrameProcessor.push_frame()` this will internally call
+  `FrameProcessor.queue_frame()` on the next processor (upstream or
+  downstream). Then, the queue frames will get processed. With this input queue
+  it is now possible for FrameProcessors to block processing more frames via a
+  list of blocking frames. For example, some TTS services now block processing
+  more frames if they see a `TTSSpeakFrame` or a `LLMFullResponseEndFrame` until
+  the bot has stopped speaking. This makes sure we don't mix audio from
+  different sentences. The way to resume processing frames is by calling
+  `FrameProcessor.resume_processing_frames()`.
+
 - Added audio filter `NoisereduceFilter`.
 
 - Introduce input transport audio filters (`BaseAudioFilter`). Audio filters can