fieldPath field

[avalanche123]

This PR buids on #119, as such the diff currently includes both.
To see only new additions, use avalanche123/xgql@no-json-if-not-requested...field-path

• extend gqlgen's goType directive
• add fieldPath and deprecate unstructured

Description of your changes

Add a new field fieldPath that deprecates unstructured and allows to be
exact in which data needs to be selected:

{
  name: fieldPath(path: "metadata.name")
  images: fieldPath(path: "spec.containers[*].image")
}

Some modification of gqlgen's code generation process was necessary to remove
the no longer used Unstructured field and enforce delegation for the "unstructured"
and "fieldPath" fields to the embedded model.PavedAccess field of generated
structs.

I have:

• Read and followed Upbound's contribution process.
• Run make reviewable to ensure this PR is ready for review.
• Added backport release-x.y labels to auto-backport this PR, as appropriate.

How has this code been tested

I've tested it both locally via playground and on a running MCP via console.

[avalanche123]

opened #121 instead