Reject duplicate credential services

[BharatDeva]

Summary

This adds a sanity check when creating scans so a target cannot include multiple credentials for the same service.

OpenVAS currently supports one credential per service for a target. With this change, duplicate services are rejected before the scan is stored and the API returns 400 Bad Request with a clear message.

Fixes #1783

Testing

$ cargo test -p greenbone-scanner-framework post_scans
running 9 tests
test post_scans_id::tests::missing_scan ... ok
test post_scans::tests::missing_scan ... ok
test post_scans::tests::duplicate_id ... ok
test post_scans::tests::internal_server_error ... ok
test post_scans_id::tests::internal_server_error ... ok
test post_scans_id::tests::already_running ... ok
test post_scans_id::tests::ok ... ok
test post_scans::tests::duplicate_credential_service ... ok
test post_scans::tests::post_scans ... ok

test result: ok. 9 passed; 0 failed; 0 ignored; 0 measured; 52 filtered out

$ cargo fmt --check -p greenbone-scanner-framework

$ git diff --check HEAD~1..HEAD