Run bridges zombienet tests on CI (#2439)

Brridges zombienet tests are non-standard - zombienet currently missing
multiple relay chains support (see e.g.
https://github.com/paritytech/zombienet/pull/796), so we need to go live
with two relay networks, their parachains + custom test runner (which
e.g. doesn't shutdown net when its tests are finished and instead waits
for both networks tests to complete). So we are stuck with native
zombienet provider => this PR is an attempt to gather everything in a
single docker container and run tests there ~Draft, because it is far
from finishing - what I want now is to see how it works on CI~

.gitlab/pipeline/build.yml

@@ -382,3 +382,21 @@ build-subkey-linux:
 #   after_script: [""]
 #   tags:
 #     - osx
+
+# bridges
+
+# we need some non-binary artifacts in our bridges+zombienet image
+prepare-bridges-zombienet-artifacts:
+  stage: build
+  extends:
+    - .docker-env
+    - .common-refs
+    - .run-immediately
+    - .collect-artifacts
+  before_script:
+    - mkdir -p ./artifacts/bridges-polkadot-sdk/bridges
+    - mkdir -p ./artifacts/bridges-polkadot-sdk/cumulus/zombienet
+  script:
+    - cp -r bridges/zombienet ./artifacts/bridges-polkadot-sdk/bridges/zombienet
+    - cp -r cumulus/scripts ./artifacts/bridges-polkadot-sdk/cumulus/scripts
+    - cp -r cumulus/zombienet/bridge-hubs ./artifacts/bridges-polkadot-sdk/cumulus/zombienet/bridge-hubs