Migrate MESSAGEix-Transport workflow from `message_data`

[khaeru]

Now that all code for MESSAGEix-Transport is in this repo, it is no longer really necessary to have the transport.yaml workflow per se in the message_data private repo, and use the mechanism of a local workflow transport.yaml dispatching the other.

Rather:

• The main workflow file can exist in message-ix-models.
• As with the "pytest" workflows, appropriate permissions/controls can be used to:
  • Ensure only first-party PRs (or selected third-party PRs) are able to dispatch the workflow.
  • Grant access to secrets used to clone other/third-party workflows.

This would then be a basis for other workflows that are similar or generalized, e.g. for BMT.