The multi-module generator does not support using a custom client

[cromoteca]

The endpoints-custom-client test module fails when using the multi-module generator as it doesn't pick up the provided custom client.

It should mimic the existing feature as implemented at https://github.com/vaadin/hilla/blob/42ea5619a0d670b39ef330be68caaf0423c37ad0/packages/java/endpoint/src/main/java/dev/hilla/frontend/TaskGenerateEndpointImpl.java#L50-L54

[platosha]

It should be OK for the initial implementation to assume that the frontend directory is one level above the generated one.

Check if frontend/connect-client.ts file exists:

• If so, use it for imports in the generated TypeScript files
• Otherwise, generate the frontend/generated/connect-client.default.ts client file and use it