Add various go template function from helm

[olblak]

Requires

• Support multiple manifest into one single file #1457

This pullrequest adds the following go template function

• toYaml
• fromYaml
• fromYamlArray

The goal is to provide more templating possibilities when using .tpl files such as

sources:
  default:
    {{ .sources.default | toYaml }}

targets:
    default:
      kind: file
      spec:
        file: "/tmp/example.yaml"

Fix #961

Test

To test this pull request, you can run the following commands:

cd pkg/core/config/
go test

Additional Information

Tradeoff

More advanced templating means more complex pipeline which are obviously harder to maintain on long term :(
This also means we lose schema validation for manifest which are not YAML compliant...

Potential improvement

Offering another configuration syntax like HCL or Cuelang for more advanced needs