Add Ci::Config::Header nodes to validate `spec:inputs`

What does this MR do and why?

Related to Backend: Create `Entry::Node` classes for `spec` (#391327 - closed)

This MR introduces Gitlab::Ci::Config::Header::Root node to validate the whole 1st document in a multi-doc CI YAML.

When a CI YAML contains 2 documents (max) we consider the first document as the "header" and the second document as the "configuration".

This MR validates the content of the header document using the Entry::Node framework we use for any CI configurations.

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