Dtrack-API with contextpath not accessible

[Zohhak]

Describe the bug

When the Dependency-Track-API is deployed behind a proxy with an added contextpath, the current master version of the depdency-track-plugin is unable to access it. Its already not working when testing the connection on the "configure jenkins"-site.
Examples:
configured Dependency-Track Backend URL: https://xyz.com/dtrack-api
expected generated test url: https://xyz.com/dtrack-api/api/v1/project
current wrong generated url: https://xyz.com/api/v1/project

with that a 404 is returned and the plugin is not usable anymore.

Environment

• Plugin Version: master 5.0.0-SNAPSHOT
• Jenkins Version: 2.387.3
• Java Version for Jenkins: 11
• Type of Jenkins Job: declarative pipeline (but should not matter)
• Dependency-Track Version: 4.8.0

To Reproduce

Steps to reproduce the behavior:

1. Setup dtrack-api with context-path
2. configure plugin with url
3. test connection
4. See error

Expected behavior

dependency-track-plugin honours context-path when building urls