Expose the template ast

[carolynvs]

Sometimes a consumer of a template needs to know what objects were used. In my case, a template can reference secret values from a secret store vault and instead of passing all possible secrets to the template only to render two of them, we use the ast to determine which are used and only retrieve those values from the vault before rendering the template.

Exposing the ast allows us to use the liquid APIs just like normal, without having to jump through hoops to build the ast ourselves using the other types exported in this library.

Checklist

• I have read the contribution guidelines.
• make test passes.
• make lint passes.
• New and changed code is covered by tests.
• Performance improvements include benchmarks. N/A
• Changes match the documented (not just the implemented) behavior of Shopify. N/A

[osteele]

Thank you for your contribution!

Do you have example code where you use this? If so, I would like to add a link to this in the documentation; it sounds like it could be useful material for users of this package.

[carolynvs]

How about I make a small godoc example and submit a PR so that it's easy to find? We are using it in Porter, so that we can determine which secrets to retrieve from vault and inject back into the config file but I think it would be easier to understand without all the extra Porter stuff going on.

Here's where we load our template and switch to using different delimiters (I'm so glad you support that!)
https://github.com/getporter/porter/blob/9ca4e22d223a20f4620aa67b9436c71dbcfa454a/pkg/config/loader.go#L89-L105

Here's where we parse the ast in the template and grab out any variables used:
https://github.com/getporter/porter/blob/9ca4e22d223a20f4620aa67b9436c71dbcfa454a/pkg/config/loader.go#L117-L139