feat: support config file namespace validation

[cramja]

Related issue

#586

cc @zepatrik

Proposed changes

Adds support for parsing configuration files containing either a list of namespace objects or a reference to a location with namespace objects.

It extends the existing functionality. Here's example usage.

### VALID
./keto namespace validate -c config.yaml config2.yaml

./keto namespace validate namespace.yaml ns2.yaml

# namespaces and configs are all validated
./keto namespace validate namespace.yaml ns2.yaml -c config.yaml

### INVALID

# interprets config.yaml as a namespace file
./keto namespace validate config.yaml

Checklist

• I have read the contributing guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security. vulnerability, I
  confirm that I got green light (please contact
  [email protected]) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further comments

I changed the way the namespace watcher works. This is because I wanted to reuse the code in watcher which determines the correct parser type and does an initial scrape of the target location. To make it possible to reuse watcher for validation, I had to leave the original source file contents inside the scraped file.

[CLAassistant]

All committers have signed the CLA.

[zepatrik]

Awesome, thanks a lot for looking into this 👍
Did you actually use the namespace watcher changes you made? I could not find how you used it but I am on my phone only so maybe overlooked it 😅

[zepatrik]

Sorry for the long time, I made a review but somehow got distracted and did not submit it 🤦
There are just some improvement ideas, but it is nearly good to go 👍

[aeneasr]

Just fyi - @zepatrik is on vacation until June 14th so reviews will probably take a bit longer :) I don't really feel that I have enough context to review the PR :/

[zepatrik]

I am really sorry about the long review times.
This looks very good, I have some ideas for further improvement.

[cramja]

Thanks for the follow up @zepatrik ! Your feedback is very thorough and I appreciate the patience as I learn some Go basics. I will address some more of the advanced suggestions over the next few days and re-request you as a reviewer when complete.

[cramja]

@zepatrik re-added you as a reviewer. Thanks again for taking the time for a thorough review. I learned a lot.

Also - I think I will keep the fail-fast strategy for namespace file validation because I think it makes sense to approach the corrections one-by-one.

[zepatrik]

Awesome, this is nearly good to go ❤️

[zepatrik]

Also - I think I will keep the fail-fast strategy for namespace file validation because I think it makes sense to approach the corrections one-by-one.

Good idea btw
Can you then please rebase/merge current master into this?

[cramja]

@zepatrik rebased and squashed together!

[zepatrik]

Awesome, thanks a lot ❤️