Allow mapping EnvConfigs

### What problem are you facing?
Say I have an environment configuration which has a common structure, but I'm creating multiple EnvConfigs of the same structure (e.g. documenting all configurations relevant to an EKS cluster).
For most use cases I just reference one of these EnvConfigs in a composition, but now I'm facing the situation, that I'm referencing multiple EnvConfigs of the same type in a single composition, overwriting fields in the context.
Instead, I'd like to map subsequent data to a different dict field.

### How could this Function help solve your problem?
function-environment-configs could allow mapping data into a sub structure.

```
apiVersion: environmentconfigs.fn.crossplane.io/v1beta1
kind: Input
spec:
environmentConfigs:
- type: Reference
ref:
name: cluster-config-one
mapping: clusterOne
- type: Reference
ref:
name: cluster-config-two
mapping: clusterTwo
```

Say clusterFullname is set in both EnvConfigs, it would then be available in two places:
.clusterOne.clusterFullname & .clusterTwo.clusterFullname