Replacing libwebp-sys crate

I created a crate that downloads pre-built libwebp binary files from Google and links to them. By doing this, we effectively avoid any potential issues with local builds, including upstream changes breaking the build.
https://github.com/zzwxh/webp_encoder

This crate currently does not meet the requirements, but if you're interested, I can continue.

Advantages:

1. No need to maintain a specific build system (cc-rs, cmake, etc.) by yourself.
2. Can automate all the work using a CI/CD pipeline.

Disadvantages:

1. Some users may not prefer binary distribution.
2. Google's provided build only supports mainstream platforms.