These are breaking changes of the Streamlink session and Plugin APIs.

Opening this as a draft for now, even though it's pretty much final, because I don't think that this has to be merged now. It simply fixes an old design flaw and is not too important and should be included with other breaking changes in the future in one go.

Two separate changes:

1. Moving Plugin.options to plugin instances

As mentioned in #5011, turning Plugin.options from a plugin class attribute into a plugin instance attribute is related to the removal of the Plugin.bind() class method in #4744 / #4768, and ideally should've been part of the 5.0.0 release. While 5.0.0 removed Plugin.bind(), the breaking changes were actually in the Streamlink session class where Plugin.bind() was called internally, and compatibility wrappers were added to the Plugin constructor, with deprecation messages for old-style parameters. This here is another set of breaking changes on the Streamlink session class and Plugin class, but without any deprecations.

Just to recap what I've said in #5011 and what I've included in the commit messages of this PR, the Plugin class previously shared a common Options instance as the Plugin.options class attribute. This class attribute was then overridden by streamlink_cli.main.setup_plugin_args() where plugin arguments were processed and added to the argparser while reading default values of the plugin arguments that were added to a new Options instance.

This design is bad for several reasons. First, this makes streamlink_cli a "soft" dependency of the Plugin and Streamlink classes, as all plugin options would otherwise be shared between all plugins on the same Options instance. Second, multiple instances of the same plugin always have the same set of options set (not really important, but still bad). And third, tests and third party projects not using streamlink_cli need to manually override/clear the Plugin.options class attribute when initializing multiple plugins, which is bad.

Now, after these changes:

• The Plugin class can receive an optional Options instance as a parameter on its constructor, which streamlink_cli uses for setting up individual plugin options. Having it in the Plugin constructor is important, because plugin subclasses need to be able to read plugin options immediately (see Twitch and TwitchAPI), and not after initialization.
• Since plugin options are not stored on plugin classes anymore, Streamlink.get_plugin_option() and Streamlink.set_plugin_option() have been removed. These methods were useful for sharing state between different parts of Streamlink, e.g. plugins and streams (see Twitch and TwitchHLSStream), but this can be done in different ways. The PR therefore also adds the keywords parameter to HLSStream.parse_variant_playlist() to be able to pass keywords to the {,Muxed}HLSStream classes. In case of the Twitch plugin, these keywords are disable_ads and low_latency which are read from the plugin options instead of session.get_plugin_option().

2. Removal of Argument.is_global

I've already had this on my radar for a while. Since the changed CLI methods already had to deal with global plugin arguments, I decided to include the removal of global plugin arguments in this PR.

The reason for the removal of global plugin arguments is simple: it adds unnecessary complexity for absolutely no gain. The value which a global plugin argument stores is already stored in the session options and should be read from there.

is_global was added when we removed individual plugin arguments for muxing subtitles, but no other global plugin arguments were ever added. The only benefit it had was being able to include a list of plugins in the docs which were making use of --mux-subtitles, as the global plugin arguments allowed for parsing this data. This additional data in the docs is now gone as well, but only affects four plugins. If we want to have a list of certain features a plugin uses, then this can be done by extending the regular plugin metadata.

What plugin implementors have to do to fix this is very simple, remove any global plugin arguments and change the option lookup, e.g. self.get_option("mux_subtitles") to self.session.get_option("mux-subtitles").