feat(webflux): Add configurable SSE endpoints to WebFlux transport (#41, #67)

Enhances WebFlux SSE transport implementation with customizable endpoint paths:
- Add configurable SSE endpoint support in both client and server transports
- Update tests to verify custom SSE endpoint functionality
- Implement builder pattern to support the new configuration options

Co-authored-by: haidao <wwulfric@gmail.com>
Co-authored-by: Harry <34418180+HarryFQG@users.noreply.github.com>
Signed-off-by: Christian Tzolov <christian.tzolov@broadcom.com>