chore(client): change OperationContext from type to interface

[nicolassanmar]

Closes #4518

🎯 Changes

Allows extending the OperationContext for added typesafety when implementing custom links.
This is done by changing it's definition from type to interface

✅ Checklist

• I have followed the steps listed in the Contributing guide.
• If necessary, I have added documentation related to the changes made.
• I have added or updated the tests related to the changes made.

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[nicolassanmar]

We can think of removing @internal from this as it is not entirely true. Users can interact with this when developing links.

[Sheraff]

I'm not saying this is a bad or good idea, but why do you need to change to interface? Wouldn't this work just as fine as extends on an interface?

type OperationContext = Record<string, unknown>
type CustomOperationContext = OperationContext & {
  foo: number
}

[nicolassanmar]

This is required, as your solution creates a new type but does not modify OperationContext.
We need to be able to tell Typescript that OperationContext has known custom properties.
This is required, as we can modify the context in the .useQuery(undefined, {trpc: { context: { ... } } }) (where it expects something of type OperationContext), and we access the context in links with op.context (which is typed OperationContext).

The only way to modify OperationContext is to extend it as an interface, like what we can do with MutationMeta or QueryMeta.

For context, see the issue here: #4518

[KATT]

I'll merge this as I generally favor interface over type -- however, we will still regard this as an internal type and break it at our own will :)