Remove `session` argument from methods that utilize `require_active_session()`

Having the session is good for resolving the correct DOM id (such as when in modules), and for sending messages to the browser.

However, some update methods to not utilize the session being sent in:

• https://github.com/rstudio/py-shiny/blob/0bc5ad07bac8dd418d27a848f0234ef6d79a2150/shiny/ui/_input_update.py#L226
• https://github.com/rstudio/py-shiny/blob/0bc5ad07bac8dd418d27a848f0234ef6d79a2150/shiny/ui/_input_update.py#L278

In addition, the session object is something that users shouldn't really be setting.

Proposal:

• Remove session from methods whose API has session for context only
  • Ex: update_*() methods, notification_{show,remove}(), etc.
• Internally use session = require_active_session(None) to retrieve the session
• Show examples of using with shiny.session.session_context(session=): to set the context. Such as in an app where a button updates two modules's checkbox group choices.

Doing this would naturally resolve the two situations above and make the API cleaner.