Added --output option to firestore:indexes command

[aalej]

Description

Add --output option to firestore:indexes command. When the --output is passed, it writes the indexes output to the specified filename, if no filename is specified, it will get the filename from firebase.json based on the db name passed from --database.

Scenarios Tested

Given the ff files:

firebase.json

{
  "firestore": [
    {
      "database": "(default)",
      "rules": "firestore.rules",
      "indexes": "firestore.indexes.json"
    },
    {
      "database": "non-default",
      "rules": "firestore-non-default.rules",
      "indexes": "firestore-indexes-non-default.indexes.json"
    }
  ]
}

firestore-indexes-non-default.indexes.json

{
  "indexes": [],
  "fieldOverrides": []
}

firestore.indexes.json

{
  "indexes": [],
  "fieldOverrides": []
}

Sample Commands

firebase firestore:indexes --output Updates firestore.indexes.json

{
  "indexes": [
    {
      "collectionGroup": "messagespaces",
      "queryScope": "COLLECTION",
      "fields": [
        {
          "fieldPath": "userIds",
          "arrayConfig": "CONTAINS"
        },
        {
          "fieldPath": "updatedAt",
          "order": "ASCENDING"
        }
      ]
    }
  ],
  "fieldOverrides": []
}

firebase firestore:indexes --output --database non-default Updates firestore-indexes-non-default.indexes.json

{
  "indexes": [
    {
      "collectionGroup": "random-collection",
      "queryScope": "COLLECTION_GROUP",
      "fields": [
        {
          "fieldPath": "a",
          "order": "ASCENDING"
        },
        {
          "fieldPath": "b",
          "order": "DESCENDING"
        }
      ]
    }
  ],
  "fieldOverrides": []
}