add Gradle Build Cache support with handler and tests

[matikepa]

• Accepts cache keys at both:
  /gradle/cache/{key}
/gradle/{key}
• PUT stores cache entries in storage backend under _gradle/http-build-cache/{key}
• GET returns stored artifact with Gradle cache content type
• HEAD returns metadata without body
• First PUT returns 201 Created; overwrite returns 200 OK
• Path traversal / invalid keys are rejected