Add missing context option to useSubscription

[jcreighton]

Fixes #7824. This PR adds context to useSubscription's options.

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests

[jcreighton]

@benjamn I noticed we're also not allowing errorPolicy to be passed into the options for useSubscription. Was that on purpose?

[benjamn]

Looking good to me!

Before merging, let's also update the documentation. Specifically, I believe we need to add the context option to this <SubscriptionOptions/> MDX component, which gets rendered in the subscription docs and in the React Hooks API docs. If you push changes to this PR branch, Netlify should automatically render the docs so you don't have to build them locally.