[ExpressionLanguage] Make ArrayNode::getKeyValuePairs method public

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

I want to get nodes from an ArrayNode instance but i get an array of keys and values instead of only values. Internally Symfony has a getKeyValuePairs method that normalize the array of nodes. Can be useful to anyone if this method is public instead of protected.