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GH-34778: [Java] Only apply ServerInterceptorAdapter logic to Flight service requests (#34815)

### Rationale for this change

The interceptor applies to all methods. When receiving a method for a different service, we don't want to throw an error because the method doesn't exist.

### What changes are included in this PR?

- Ensure added services work correctly
- Add test to make sure registered services work correctly
- Add documentation explaining how to add services

### Are these changes tested?

Yes

### Are there any user-facing changes?

Yes, documentation now explains how to add services

* Closes: #34778

Authored-by: Diego Fernandez <aiguo.fernandez@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>