fix: handle missing optional dependency in declarations

[myandrienko]

This package has an optional dependency: @stream-io/openapi-realtime-api. We import types from this package, but usually this is not a problem, because:

1. We import types with import type statement, so this import doesn't affect the runtime
2. Most setups have skipLibCheck: true in their tsconfig, so importing types from a missing package doesn't raise errors

However, this is still a problem with skipLibCheck: false. It seems the only way to fix this is to add @ts-ignore to the import statement in declaration file.