Add deploymentTimestamp and firstTimeFullySyncedTimestamp to SubgraphIndexingStatus

It would be great for subgraph developers to find out how long their subgraph took to fully sync for the first time. This could be enabled by changing the type SubgraphIndexingStatus here to the following shape:

type SubgraphIndexingStatus {
  subgraph: String!
  synced: Boolean!

  # Note that the health can be implied from fatalError and nonFatalErrors:
  # - If fatalError is non-null, then health is 'failed'.
  # - Else if nonFatalErrors is non-empty, then health is 'unhealthy'.
  # - Else health is 'healthy'.
  health: Health!

  "If the subgraph has failed, this is the error caused it"
  fatalError: SubgraphError
  "Sorted from first to last, limited to first 1000"
  nonFatalErrors: [SubgraphError!]!
  chains: [ChainIndexingStatus!]!
  entityCount: BigInt!
  node: String
  deploymentTimestamp: BigInt! # <- Added 
  firstTimeFullySyncedTimestamp: BigInt # <- Added, not required because it's empty during sync
}

Start with the SubgraphIndexingStatus definition in server/index-node/src/schema.graphql and trace where its values are assembled. Check how deployment and first full synchronization times can be obtained, then expose both fields with the requested required and nullable behavior. Done means the schema and its responses include accurate timestamps, with the first-sync field empty during synchronization.