Make extension scoping work with only one field

[DMRobertson]

Fixes #148.

[DMRobertson]

I think some of this ugliness comes from the fact that we use the same struct type to represent a blob of data D and an optional change to that blob of data.

[kegsay]

I don't know what this is supposed to do.

[DMRobertson]

See the comment above the declaration of this in the interface. https://github.com/matrix-org/sliding-sync/pull/149/files#r1232127907

To illustrate: suppose I receive a request whose account data extension is configured with

{
   "enabled": "true"
}

If this is a brand-new request, the proxy should process this in two steps:

1. Use the initial sticky parameters for lists and rooms (i.e. ["*"]).
2. Set enabled to true

If this is a follow-up to a previous request, the proxy should interpret this as follows

1. Use the previous sticky values for lists and rooms.
2. Set enabled to true.

The way I wanted to handle this was something like:

• On GenericRequest, define a Default() method that returns a new GenericRequest of the same type, filled with initial sticky values.
• Use this in Request.ApplyDelta to generate curr in the brand-new case when isNil(curr). Then call curr.ApplyDelta(next), just like we already do in the follow-up case.

The problem is that I couldn't figure out a way to do the first bullet. It felt like I was fighting the language. So instead, InterpretAsInitial has some extra logic to mutate next in place. The intention is that
Default().ApplyDelta(next) would be equal to next after running next.InterpretAsInitial().

[DMRobertson]

FTAOD I'm not very happy with InterpretAsInitial. If there is some nice way to implement Default() or similar I'm all ears.

[kegsay]

Can we actually test the EX use case as well please as another test? I.e rooms: [*], lists: [] on the receipts extension -> only see receipts for subscribed rooms.