Support a read-only mode on the engine

[Kerollmops]

Hey,

After a meeting with the @meilisearch/cloud-team we concluded that it would be great to set a Meilisearch in read-only mode, making it readable but refusing any write request. This feature is useful in the update process when the cloud team is doing a green-blue deployment.

It would be great to also be able to enable the read/write mode too. The process of changing the mode of the engine could be to send an HTTP request to a specific HTTP route with the necessary rights (master key only?).