ci(flutter_agy_docker): conditionally tag docker images by release channel (#5086)

Add `orchestrator.yaml` to fetch the latest `stable` Flutter version and automatically trigger the `flutter-agy-docker` build with the correct version substitution.

Update `cloudbuild.yaml` to fetch the official Flutter releases JSON and conditionally apply docker tags:
- Tag and push as `stable` and `latest` if the built version matches the current stable release.
- Tag and push as `beta` if the built version matches the current beta release.
- Remove the unconditional `latest` tagging that applied to every build.

Flows:

```mermaid
sequenceDiagram
    autonumber
    actor Scheduler as Cloud Scheduler
    actor User as Manual Trigger
    participant Orchestrator as orchestrator.yaml
    participant CloudBuild as cloudbuild.yaml
    participant Storage as Google Cloud Storage (Releases)
    participant AR as Artifact Registry

    %% Cloud Scheduler Flow
    rect rgb(240, 248, 255)
        note over Scheduler, AR: Flow 1: Automated Stable Build
        Scheduler->>Orchestrator: Trigger daily/weekly
        Orchestrator->>Storage: Fetch releases_linux.json
        Storage-->>Orchestrator: Return JSON
        Orchestrator->>Orchestrator: Parse latest stable version (e.g. 3.44.4)
        Orchestrator->>CloudBuild: gcloud builds triggers run <br/>(_FLUTTER_VERSION=3.44.4)

        CloudBuild->>CloudBuild: Build images (flutter_base, flutter_docker)
        CloudBuild->>Storage: Fetch releases_linux.json (Step 3)
        Storage-->>CloudBuild: Return JSON

        CloudBuild->>CloudBuild: Compare _FLUTTER_VERSION to stable/beta
        note right of CloudBuild: Matches Stable

        CloudBuild->>AR: Push tag: :3.44.4
        CloudBuild->>AR: Push tag: :stable
        CloudBuild->>AR: Push tag: :latest
    end

    %% Manual Trigger Flow
    rect rgb(255, 248, 240)
        note over User, AR: Flow 2: Manual Version Build
        User->>CloudBuild: Trigger manually <br/>(_FLUTTER_VERSION=X.Y.Z)

        CloudBuild->>CloudBuild: Build images (flutter_base, flutter_docker)
        CloudBuild->>Storage: Fetch releases_linux.json (Step 3)
        Storage-->>CloudBuild: Return JSON

        CloudBuild->>CloudBuild: Compare _FLUTTER_VERSION to stable/beta

        alt Matches Stable Version
            CloudBuild->>AR: Push tag: :X.Y.Z
            CloudBuild->>AR: Push tag: :stable
            CloudBuild->>AR: Push tag: :latest
        else Matches Beta Version
            CloudBuild->>AR: Push tag: :X.Y.Z
            CloudBuild->>AR: Push tag: :beta
        else Neither (Random Version)
            CloudBuild->>AR: Push tag: :X.Y.Z
            note right of CloudBuild: Skips stable, latest, and beta tags
        end
    end
```