Add basic support for cookie manager

[novelhawk]

Adds basic support for accessing cookies from webview2.

• Added complete Cookie implementation.
• Added complete CookieList implementation.
• Added 3 out of 8 CookieManager methods.
• Added get_webview2 to WebView
• Added get_cookie_manager to WebView_2 (1 out of 7)

[blckngm]

Hi, thanks for the interest for this crate. I heard that Microsoft has made windows-rs, maybe it'll work for webview2 too? If it's the case this crate is no longer that useful.