[Infra updates] Add a workflow to run the model on PRs and workflow_dispatch

[jeancochrane]

Building on #22, we need a GitHub Actions workflow that can run the model. The workflow should:

• Depend on the workflow that pushes the Docker image to the GitHub Container Registry in [Infra updates] Build and push a Docker image for the model #22
• Run on:
  • Every commit to every pull request
  • The workflow_dispatch event
• Deploy to an environment called staging that requires manual approval
• Define a job to run the model

There are two ways we could define a job to run the model. Try option 1 first, and fall back to option 2 if CML doesn't work as advertised.

Option 1: Use CML self-hosted runners

• Define a job, launch-runner, to start an AWS spot EC2 instance using cml runner
  • Set sensible defaults for the instance options, but allow them to be overridden via workflow inputs
• Define a job, run-model, to run the model on the EC2 instance created by CML
  • Set the runs-on key for the job to point at the runner
    • This will cause steps defined in the job to run on the remote runner
  • Run the model using dvc pull and dvc repro

Option 2: Write custom code to run model jobs on AWS Batch

• Run Terraform to make sure an AWS Batch job queue and job definition exist for the PR
  • The job definition should define the code that will be used to run the model itself, e.g. dvc pull and dvc repro
• Use the AWS CLI to submit a job to the Batch queue
• Use the AWS CLI to poll the job status until it has a terminal status (SUCCEEDED or FAILED)
  • Once the job has at least a RUNNING status, use the jobStreamName parameter to print a link to its logs

Depends on #22.