[snapshot] support multiple credentials in elasticsearch.yml

[dadoonet]

Based on elastic/elasticsearch-cloud-azure#93 (comment)

We need to support in elasticsearch.yml multiple credentials.
We could imagine that as a generic feature whatever repository type you want to use.

Let say that we can now create something like:

cloud:
    azure:
        storage:
            azure1:
              account: your_azure_storage_account1
              key: your_azure_storage_key1
              default: true
            azure2:
              account: your_azure_storage_account2
              key: your_azure_storage_key2
            azure3:
              account: your_azure_storage_account3
              key: your_azure_storage_key3

Then when we create the repo, we can specify which credentials we want to use:

# use credentials 2
PUT _snapshot/my_backup2
{
  "type": "azure",
  "settings": {
      "creadentials": "azure2",
      "container": "backup_container",
      "base_path": "backups"
  }
}

# This one will use the one marked as "default"
PUT _snapshot/my_backup3
{
  "type": "azure"
}

[dadoonet]

This has been fixed by #13779