`apibara up` command to run multiple indexers at the same time

Is your feature request related to a problem? Please describe.

At the moment, if I want to run multiple indexers concurrently, I need to create many terminal windows and run each one of them with apibara run my-indexer.ts. If I change my code, I then need to restart them individually.

Describe the solution you'd like

We should have a configuration file (apibara.config.ts) where I can define all indexers I want to run. Then there should be a command apibara up that reads this configuration file and starts the indexers defined there.

The configuration file should be in typescript/javascript so that developers can read environment variables and generate the indexers configuration at runtime.

export default {
  indexers: {
    "indexer-1": {
      script: "src/indexer1.ts",
      env: {
       "KEY_1": "value_1"
     }
    },
    "inedxer-2": {
      script: "src/indexer2.ts",
    }
  },
  persistence: {
    fs: ".apibara",
   // etcd: "http://localohst:2379"
  }
}

Notice that the key of the indexers property will be used as the sink-id if persistence is enabled.

This feature requires some planning:

how to forward environment variables to the sub-processes? At the moment --allow-env only accepts files.
- different indexers may have different values for the same env variable. How?
- One idea is to enable a syntax like --allow-env=KEY=VALUE1,KEY2=VALUE2
run multiple indexers concurrently as sub-processes. forward signals to the sub-processes (is this automatic?)
assign a different status server address to each sub-process, and create a new status server that is healthy only if all sub-processes are healthy

Describe alternatives you've considered

We could use yaml/json for configuration. I believe using typescript is better because we get the following for free:

autocomplete
type safety
powerful interpolation/templating

Additional context

This is related to the work on factory indexers.

apibara / dna

`apibara up` command to run multiple indexers at the same time #226