feat(parser): support ref-tags with default values

[MatteoVoges]

Proposed Changes

• Added support for specifying default values for non existing keys / dictionary-paths
• Extended functionality to support a reference-tag as default value, e.g. ${spec:value||${default:value||another-default}}
• Use || as delimiter, because it fits well in the kapitan ecosystem

Examples

parameters:
  existing:
    key: myvalue

  foo: ${non:existing:key||bar} # --> bar (default value)
  mykey: ${existing:key||bar} # --> myvalue (value exists)

parameters:
  defaults:  
    default1: myDefault

  spec:
    spec1: ${not:existing||${defaults:default1}} # --> myDefault (tag in default-value exists)
    spec2: ${not:existing||${defaults:notfound||fallback}} # --> fallback (tag in default-value not exists, default gets taken) 

Breaking changes

• You should not have the || - delimiter in your yaml-keys anymore!

To Do

• Add tests
• Add documentation (also in kapitan docs)
  @ademariag Where should this be done?

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