function overrides the whole context (key) instead of inserting new (`into`) keys

### What happened?

The function overrides all previous runs instead of adding new keys to the context.

In the case below, I would expect the 2nd run to simply add `obj-1` into the context, and leave `obj-0` be, so that both `obj-0` and `obj-1` are present at the end.

### How can we reproduce it?

```yaml
- step: first
functionRef:
name: crossplane-contrib-function-extra-resources
input:
apiVersion: extra-resources.fn.crossplane.io/v1beta1
kind: Input
spec:
policy:
resolution: Required # these are required
extraResources:
- into: obj-0
# ...
- step: second
functionRef:
name: crossplane-contrib-function-extra-resources
input:
apiVersion: extra-resources.fn.crossplane.io/v1beta1
kind: Input
spec:
policy:
resolution: Optional # these are optional
extraResources:
- into: obj-1
# ...
```

### Suggested solution

Load the potentially existing context and insert the new (`into`) keys. No need to deep-merge.