Add Global Attributes, Global Variable & Substitution and Deprecate Metadata.Attributes

[maysunfaisal]

Signed-off-by: Maysun J Faisal maysunaneek@gmail.com

What does this PR do?

• Introduces a global attribute property (not a top level list since Attribute is a free form yaml map and does not have name/id)
• Implements Attribute overriding and merging as per the current devfile overriding and merging implementation for other fields
• Implements substitution for global attributes
  • global attribute is referenced by {{attributeKey}} where attributeKey is the key entry in the global attribute
  • valid for all string fields in the devfile except for schemaVersion & metadata
  • exceptions are top level keys like name/id as they're guarded by regex and also required for overriding & merging and string enums as they're guarded by schema validation
  • exceptions also include int & boolean properties, as fields for these properties cannot accept {{}} as value in the devfile

📌 EDIT POST DEVFILE CABAL AND PR REVIEW DISCUSSION:

Post Devfile cabal call & PR review discussion, the above criteria has been modified, Please refer #352 (comment) for the PR acceptance criteria

What issues does this PR fix or reference?

Fixes #239

Is your PR tested? Consider putting some instruction how to test your changes

Yes, updated current tests for overriding & merging and added new go tests.

Docs PR