Add omit_unreferenced_implementations config option (#416)

[WillAbides]

Closes #416

Adds an opt-in omit_unreferenced_implementations config option that collapses unfragmented interface/union implementations into a single catch-all struct per interface selection.

With omit_unreferenced_implementations: true:

• Per-type structs are generated only for implementations actually referenced via an inline or named fragment in the selection set.
• A single catch-all struct <…>GenqlientOther is generated for each interface/union selection. It carries the interface's shared fields and satisfies the generated Go interface like any other impl. I chose the "GenqlientOther" suffix because it is very unlikely to be used in a real query.
• Defaults to false for backward compatibility.

Tests

• Snapshot tests for the inline-fragment, named-fragment-on-interface, named-fragment-on-concrete-type, union, abstract-fragment-spread-inside-concrete-inline-fragment, and all-impls-collapse cases. The Build subtest confirms each snapshot compiles.
• Integration test in internal/integration/omitimpls/ that exercises both the typed-impl path and the catch-all path end-to-end against the existing gqlgen test server.

I have:

• Written a clear PR title and description (above)
• Signed the Khan Academy CLA
• Added tests covering my changes, if applicable
• Included a link to the issue fixed, if applicable (Interfaces with many implementations generate massive output even when only one type is queried via fragment #416)
• Included documentation, for new features
• Added an entry to the changelog