Allow configurable tag name overrides in native types

[matthchr]

Allowing tags other than "cel" enables more flexibility, particularly when the types in question aren't owned by those configuring the CEL environment.

The main use-case for this is enabling an experience in CEL that matches the shape of an object in JSON (or YAML). This includes:

• Property casing: 'Property' in Go would likely be 'property' in JSON/YAML.
• Embedded structures: metav1.ObjectType json:"metadata,omitempty" is common on Kuberentes resources, allowing users to request CEL use the JSON tags enables access via the ".metadata" property even though there is no such property on the actual Native type.

Reviews

• Perform a self-review.
• Make sure the Travis CI build passes.
• Assign a reviewer once both the above have been completed.

Merging

• If a CEL maintaner approves the change, it may be merged by the author if they have write access. Otherwise, the change will be merged by a maintainer.
• Multiple commits should be squashed before merging.
• Please append the line closes #<issue-num>: description in the merge message, if applicable.

[google-cla[bot]]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[TristonianJones]

Hi @matthchr, thanks for the contribution. Would you mind signing the CLA? I'll try to give this a review by the end of the week.

/gcbrun

[matthchr]

I've signed the CLA. Let me know if you have any questions as well - I'm happy to talk through our use-case. I had originally done a different approach, something like this:

type NativeFieldAdapter = func(rt reflect.Type, name string) (reflect.StructField, bool)

func defaultNativeFieldAdapter(rt reflect.Type, name string) (reflect.StructField, bool) {
    return rt.FieldByName(name)
}

that users could configure, before I realized that actually all I really needed was to be able to configure the tag to parse for the feature that already existed, because what I wanted to do was use JSON tags rather than a new CEL tag.

[TristonianJones]

/gcbrun

[TristonianJones]

/gcbrun

[matthchr]

Thanks for merging!