Add getter/setter for global_header

[daveisfera]

If there's not a better option, then this is the fix for #1814 (creating this so I can test a fully integrated build)

[WyattBlue]

This is not necessary, you can already read/write ptr.flags:

from av.codec.context import Flags

video.codec_context.flags |= Flags.global_header

Even if this is a common operator, your PR would need proper type hints and a basic test.

[daveisfera]

Awesome! This is what I was looking for and couldn't figure out.

Writing down how to clear the flag in case other stumble on this:

video.codec_context.flags &= ~Flags.global_header