Support binding generation with system FFmpeg

[ldm0]

Fixes #23 .
Adds a new CI job that tests binding generation with FFmpeg install on system.
And currently binding generation procedure is like:
First check if is docs.rs docuement generation, if it is, use special process.
Then check if PKG_CONFIG_PATH is set:

• If it's set, use the package-config files in that path to generate bindings.
• If not, first check if system FFmpeg exists.
  • If exists, use the system FFmpeg to generate bindings.
  • If not, git clone the newest FFmpeg from GitHub and build it.