Add support for Go template expressions in PublishedResources

[xrstf]

Summary

This PR completes what we begun in #44: it adds support for Go templates in various places in a PublishedResource.

Please see the attached updated documentation for more information on how templating works.

In the old agent default, we used variables like $remoteNamespaceHash to construct object names. However these hashes were very limited:

• fixed length
• SHA-1 (a broken algorithm)

I saw this PR as a good opportunity to clean this up. So I changed the defaults to using SHA-3 instead, because I made it possible for users to keep their old naming rules by using the shortHash function in the agent.

So if you didn't configure any naming rules before, you will now need to either accept that objects will be placed in new locations (because the hashes will change) or you need to explicitly configure naming rules using the shortHash function to mimic the old default.

---

This PR also contains a clarification: In #44 we said

If a reference points to a non-existing field in an object, it will not be treated as an error, but simply as "does not exist yet".

This was however not true from the code. This PR adjusts the code to actually make seemingly-broken references work as documented.

What Type of PR Is This?

/kind feature

Related Issue(s)

Fixes #46

Release Notes

* Add support for Go templates in various places in a PublishedResource.
* Deprecate `$variable` based naming in favor of Go templates.
* ACTION REQUIRED: The default value for naming rules has changed to use SHA-3 instead of SHA-1 by default.

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[xrstf]

/retest

[embik]

Two minor nits/questions. Overall awesome PR.