Add `Loader::MultiDocYaml` to load multi-doc YAML

What does this MR do and why?

This is the first MR for #383375 (closed)

The upcoming CI Catalog feature set includes an update to CI configuration files that requires the use of multi-document YAML. The existing Loader::Yaml class only accepts single document YAML, since YAML.safe_load does not work for multi-document files.

This MR adds a new class Loader::MultiDocYaml to split a multi-document file into single documents, and then safely loads each document.

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