feat: add helper method to retreive extra resources

[twobiers]

Description of your changes

This PR adds a simple helper function to retrieve an extra resource from the request.
Previously one had to write something like this in the template to retrieve a single resource:

{{- $bucket := mustFirst (index (index .extraResources "bucket") "items") }}

With the helper function it simplifies to:

{{- $bucket := mustFirst (getExtraResources "bucket") }}

I have:

• Read and followed Crossplane's contribution process.
• Added or updated unit tests for my change.