Add CI workflow releasing Fleet charts against a test repo

[weyfonk]

This adds new workflow release-against-test-charts.yml, which, when triggered manually:

1. Builds Fleet images and Rancher-compatible Fleet charts from the provided ref
2. Pushes Docker images to ttl.sh and charts to a feature branch on the specified test charts repo, using the specified rancher/charts branch as its base.

Created Fleet charts use hard-coded version 999.9.9+up9.9.9 to prevent collisions with existing versions on the base rancher/charts branch.

Refers to #1640.

This has been tested by:

1. Running this workflow against weyfonk/fleet's master, with default rancher/charts branch dev-v2.9 and default test charts repo fleetrepoci/charts (see output)
2. Building Rancher 2.9 locally from branch release/v2.9
3. Installing Rancher in a local k3d cluster as follows:

helm upgrade --install rancher rancher-latest/rancher --devel \
     --namespace cattle-system --create-namespace \
     --set hostname=<url> \
     --set bootstrapPassword=admin \
     --set replicas=1 \
     --set rancherImage=test-tan/rancher --set rancherImageTag=dev \
     --set "extraEnv[0].name=CATTLE_CHART_DEFAULT_URL" --set "extraEnv[0].value=https://github.com/fleetrepoci/charts" \
     --set "extraEnv[1].name=CATTLE_CHART_DEFAULT_BRANCH" --set "extraEnv[1].value=fleetci-dev-v2.8-20240405112731" \
     --set "extraEnv[2].name=CATTLE_FLEET_VERSION" --set "extraEnv[2].value=999.9.9+up9.9.9"