fix(composition): emit implements on type definition instead of extend type

[briannafugate408]

The Rust composition implementation was emitting object types that implement interfaces in a split form. So the SDL had a main type definition without implements and a separate extend type implements block. In merge_implements, we were passing the source subgraph’s ComponentName into insert_implements_interface. When a subgraph had extend type Foo implements I, that name had Extension origin, so the merged schema’s “implements” was attached to an extension component and printed as a separate extend type ... implements ....

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[dariuszkuc]

instead of all those asserts why not just have an expected composed schema and assert using insta?

e.g.

assert_snapshot!(supergraph.schema().schema());