feat: move gang scheduling metadata to typed protobuf fields

[dejanzele]

What type of PR is this?

Enhancement / Bug fix

What this PR does / why we need it

Moves gang scheduling metadata from string annotations to typed protobuf fields, improving type safety and clarifying the scheduler<>executor contract.

New types

Added three new types to replace annotation-based gang scheduling:

• GangInfo represents the user's gang configuration (gang ID, cardinality, and node uniformity label name). Users submit this in the new SubmitJob.gang field.
• GangPlacement extends GangInfo with the scheduler's placement decision. After the scheduler decides where to place the gang, it adds the node_uniformity_label_value field. This complete placement info gets sent to executors.
• SchedulingMetadata is a container message in JobRunLease that holds the GangPlacement. This gives us room to add other scheduling decisions in the future.

Server

The server now handles gang metadata in two phases:

• During job submission, it validates the new gang field or converts legacy gang annotations into SchedulingMetadata. The buildSchedulingMetadata() function handles both cases, so old and new clients both work.
• During scheduling, the scheduler populates the node_uniformity_label_value based on where it decides to place the gang. It then sends the complete SchedulingMetadata to the executor via JobRunLease.

Executor

Executor now receives fully-populated SchedulingMetadata from the scheduler and uses it directly to build environment variables for the Armada jobs(pods).

Example

queue: test-queue
jobSetId: gang-example
jobs:
  - priority: 0
    namespace: default
    gang:
      gangId: my-distributed-training
      cardinality: 3
      nodeUniformityLabelName: kubernetes.io/hostname
    podSpec:
      containers:
      - name: main
         image: alpine
         command: ["sleep", "5"]
         resources:
           requests:
             cpu: 1
             memory: 1Gi
           limits:
             cpu: 1
             memory: 1Gi