Add support for types to implement imported interfaces

[mbryzek]

This PR adds support for a type to extend an interface defined in another service.

See the spec: UnionWithImportedInterfaceSpec for an example

Test resources are identical except for whitespace (fixing a few instances where we had redundant new lines)

Tests are passing locally:

[info] Run completed in 14 seconds, 963 milliseconds.
[info] Total number of tests run: 386
[info] Suites: completed 47, aborted 0
[info] Tests: succeeded 386, failed 0, canceled 0, ignored 0, pending 5
[info] All tests passed.
[success] Total time: 15 s, completed Oct 9, 2025, 8:39:49 PM