Two-phase pipeline parsing

[DrJosh9000]

There's a bug where multiple map merges aren't unmarshaled into structs in the way we've supported them.

To address this we need to go back to having greater control over the YAML unmarshaling process. It will also help if we, say, want to switch YAML library later on - if a new library supports the things we need to do, then we can simplify, and if it doesn't (or has subtle differences) then the intermediate processing can be the place where we make up the gap.

The key change here is from allowing yaml.v3 to unmarshal into structs using its reflective logic to:

• Use yaml.v3 to parse into *yaml.Node, which provides all the relevant information in the document without further processing.
• Unmarshal that node using ordered, which handles merges and aliases (including multiple merges).
• Unmarshal the dynamic, weakly-typed structure made up of *ordered.MapSA, []any, and basic types into each struct manually.

The last manual part could be converted into a yaml.v3-like or encoding/json-like reflection-based approach at some point.

[triarius]

Amazing stuff!

[moskyb]

fantastic work 🎉