Simplify stream capture and remove thread

[edgarriba]

This pull request includes significant refactoring and simplification of the StreamCapture implementation in the kornia-io crate, as well as minor changes in several example files to remove unnecessary mutability.

Refactoring and simplification of StreamCapture:

• Removed the use of Arc and Mutex for last_frame and simplified the StreamCapture struct by replacing last_frame and handle with appsink (crates/kornia-io/src/stream/capture.rs, crates/kornia-io/src/stream/capture.rsL1-R8).
• Simplified the start method by removing the thread handling and using bus.set_sync_handler to handle bus messages (crates/kornia-io/src/stream/capture.rs, crates/kornia-io/src/stream/capture.rsL38-R47).
• Updated the grab method to use appsink.try_pull_sample instead of accessing last_frame (crates/kornia-io/src/stream/capture.rs, crates/kornia-io/src/stream/capture.rsL106-R85).
• Simplified the extract_image_frame method by directly using the sample parameter instead of pulling it from appsink (crates/kornia-io/src/stream/capture.rs, crates/kornia-io/src/stream/capture.rsL163-R107).

Removal of unnecessary mutability in example files:

• Removed mut from webcam and capture variable declarations in multiple example files (examples/features/src/main.rs, examples/filters/src/main.rs, examples/rtspcam/src/main.rs, examples/video_write/src/main.rs, examples/webcam/src/main.rs, [1] [2] [3] [4] [5].