Add updateAt and createdAt in the index class

[curquiza]

When manipulating indexes (get/create/update), the SDKs return an instance of Index which already contains the primaryKey attribute.

ex:

index = client.index('movies')
index.primary_key

But we cannot do:

index.updated_at
index.created_at

We should be able to do this so that every information are available from the index instance.

TODO

• meilisearch-dart
• meilisearch-dotnet
• meilisearch-go
• meilisearch-java
• meilisearch-js
• meilisearch-php
• meilisearch-python
• meilisearch-ruby
• meilisearch-rust
• meilisearch-swift

[curquiza]

Additionnaly to the Python SDK, it's already implemented in:

• Dart SDK
• Java SDK
• Swift SDK

And partially implemented in

• JS SDK (for example, the attributes are not updated in getRawInfo)