`PerformJoin` can't use `fclient.FederationClient` as `FederatedJoinClient`

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This is due to the `MakeJoin` and `SendJoin` functions returning pointer-less responses. To work around this, one must do something like the following:

```go
type internalJoinClient struct {
upstream fclient.FederationClient
}

func (c *internalJoinClient) MakeJoin(ctx context.Context, origin spec.ServerName, via spec.ServerName, roomId string, userId string) (gomatrixserverlib.MakeJoinResponse, error) {
res, err := c.upstream.MakeJoin(ctx, origin, via, roomId, userId)
return &res, err
}

func (c *internalJoinClient) SendJoin(ctx context.Context, origin spec.ServerName, via spec.ServerName, event gomatrixserverlib.PDU) (gomatrixserverlib.SendJoinResponse, error) {
res, err := c.upstream.SendJoin(ctx, origin, via, event)
return &res, err
}
```

then, the `PerformJoin` function can be used as such:

```go
client := fclient.NewFederationClient( /* ... */ ) // or wherever you get your client from
join, err := gomatrixserverlib.PerformJoin(ctx, &internalJoinClient{upstream: client}, gomatrixserverlib.PerformJoinInput{ /* ... */ })
// etc
```

Start with the PerformJoin entry point and the FederatedJoinClient interface, then compare their MakeJoin and SendJoin signatures with fclient.FederationClient. Confirm the pointer-versus-value response mismatch and inspect any related interface tests. Done means a FederationClient can be passed to PerformJoin without the adapter shown in the issue.

Written by the indexing model from the issue text.