[Context] add learnings from the front

[Westbrook]

Some teams (e.g. the Lit team) have been registering learnings around the Context Protocol of that are not currently included in the documented version. Let’s get those included back via PRs!

Some features I know about:

• lazy context registration
• late changing context hosts
• I’m sure there are more

@justinfagnani can you help get the most up-to-date info on this?

[augustjk]

I do want to get around to making a PR to update the protocol with some things @lit/context did. Leaving some notes here while it's on my mind:

• allow late loading components to begin or take over providing context, which can be done in a couple ways
  • attaching a ContextRoot which collects unsatisfied context requests
  • allowing a context provider to relinquish its subscriptions to a late-loaded provider between itself and the consumer [labs/context] Late loaded elements can take over the context subscriptions for their children lit/lit#4014
• in order to facilitate above use cases, a context-provider event is dispatched by context providers on being connected, to be listened for by the ContextRoot and ContextProvider.
• re-parenting subscriptions require keeping track of the consumer host element. @lit/context used to use event.composedPath()[0] for this but it was reported that this can be costly, so we've added the contextTarget property to the ContextRequestEvent for keeping track of it. perf(context): skip expensive computation of Event.composedPath() lit/lit#4734 (comment)