Add a helper function to access composition environment variables

### What problem are you facing?

Currently, we can access composition environment variables like:
```
env: {{ (index .context "apiextensions.crossplane.io/environment").key1 }}
```
However, this is too verbose and can be simplified with a helper function.

### How could this Function help solve your problem?

To make it easier to access composition environment variables, we can have a helper function called `getCompositionEnvVar`. The interface might be like:

```
env: {{ getCompositionEnvVar . "key1" }}
```

The issue names no files, tests, or entry points. Start by locating how template helper functions are registered and how composition environment values are exposed, then compare the proposed `getCompositionEnvVar . "key1"` call with the existing `.context` access. Done means the helper works for the shown key and has coverage for its expected behavior.

Written by the indexing model from the issue text.