Add examples crate

[Dinnerbone]

Four examples are included with this PR, with a framework to add more:

• wgpu_frame_images: Demonstrates capturing frame images with wgpu
• secondary_frames: Demonstrates secondary frames, both continuous and discontinuous
• zones: Demonstrates the use of zones to measure work
• threads: Demonstrates the use of zones across threads

Run them with cargo run -p tracy-client-examples -- example_name.

[Dinnerbone]

Looks like wgpu has a higher rustc requirement - can we bump ours?
(or is there a way to have that separate from examples project?)

[nagisa]

I'll fix the CI before a release, thank you very much!