Ignore unknown root elements starting with x-

[jetersen]

This should help #1246 to use anchors on root level.

Added tests for unknown root elements and added an alias test for EC2 cloud to demonstrate the potential.

cc @v1v @timja @almiroshnich

fixes #1246

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[jetersen]

This would properly need some docs

[timja]

Needs docs and 1 question on naming, but nice!

[codecov]

Codecov Report

Merging #1379 into master will increase coverage by 0.19%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #1379      +/-   ##
============================================
+ Coverage     79.42%   79.61%   +0.19%     
- Complexity      797      804       +7     
============================================
  Files            66       66              
  Lines          2323     2330       +7     
  Branches        323      325       +2     
============================================
+ Hits           1845     1855      +10     
+ Misses          377      375       -2     
+ Partials        101      100       -1     

Impacted Files	Coverage Δ	Complexity Δ
...a/io/jenkins/plugins/casc/ConfigurationAsCode.java	75.47% <100.00%> (+1.31%)	101.00 <5.00> (+7.00)

[jetersen]

@almiroshnich any suggestion on how to document this feature?

[sun-mir]

@jetersen huge kudos to you for doing this 🚀

As for the documentation subject, I don't feel myself competent enough here to make suggestions, but if you'd ask me directly - I'd put a note about such functionality under jenkinsci/configuration-as-code-plugin//README.md#initial-configuration (after unclassified screenshot, before Examples section). Such a note could have a reference to the official YAML Node Anchors documentation and a few links to the existing JCasC demos (have to be adjusted as well). One could be easily taken from your test here or from my GH issue. Probably, yet another reasonable use of such functionality with anchors might be for permanent build agents, but practically speaking it's pretty much the same as other cloud-templates, either eC2Fleet or kubernetes...

[v1v]

neat! From the docs point of view, in the main README.md, I think it could be added as an example of using the node anchors, to be automatically validated with the annotation: @ConfiguredWithReadme("README.md#id") as done in the https://github.com/jenkinsci/configuration-as-code-plugin/blob/master/integrations/src/test/java/io/jenkins/plugins/casc/TopReadmeTest.java.

[jetersen]

as a FYI, we use snakeyaml at the moment which is a processor for YAML 1.1 spec.

[jetersen]

I would like to avoid linking to the yaml spec as we do not follow the spec 100% for instance we do not support maps.

[jetersen]

As this has full test coverage and blocking #1375 I am going ahead and merging it.