feat(webmvc): Add support for custom context paths in WebMvcSseServerTransportProvider

Adds the ability to specify a base URL for message endpoints in WebMvcSseServerTransportProvider,
enabling proper handling of custom servlet context paths in Spring WebMVC applications.
This ensures that clients receive the correct full endpoint URL when connecting through SSE.

- Add messageBaseUrl field to WebMvcSseServerTransportProvider
- Create new constructor that accepts messageBaseUrl parameter
- Update endpoint event to include base URL in the message endpoint
- Add TomcatTestUtil class to simplify test server creation
- Add WebMvcSseCustomContextPathTests to verify custom context path functionality
- Refactor WebMvcSseIntegrationTests to use the new TomcatTestUtil

Co-authored-by: Christian Tzolov <christian.tzolov@broadcom.com>
Signed-off-by: Christian Tzolov <christian.tzolov@broadcom.com>