Support providing Go templates as OCI images

### What problem are you facing?

Currently, the function can get template inputs as inline or from a file system directory. As a next step, we can support getting them from OCI images. In this way, users may easily decide which version of the templates to use in their composition.

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

Following API is suggested by @turkenh in the [comment](https://github.com/crossplane/crossplane/pull/4935#discussion_r1391039696):

```
input:
apiVersion: gotemplating.fn.crossplane.io/v1beta1
kind: GoTemplate
source: Image
image:
repo: turkenh/my-templates
tag: v0.1.0
```
In this case, templates are expected in a fixed path like `/templates`.

Start by examining how the function currently accepts inline templates and templates from a filesystem directory. Then trace the proposed source: Image API and determine how a versioned image would provide templates from the fixed /templates path. Done means users can select a tagged OCI image and the function loads its templates successfully.

Written by the indexing model from the issue text.