ec_deployment: Elasticsearch snapshot_settings management

[marclop]

Description

Currently, the ec_deployment resource has an elasticsearch.snapshot_settings object which can be used to manage the Elasticsearch snapshot settings for a given deployment.

The snapshot settings management is currently not implemented and when doing so, it should be done with care.

Managing the default ESS repository

When a Deployment lives in ESS, it automatically has an assigned S3 repository for the Elasticsearch resource of the deployment.
The configuration is readable when the remote resource is read and the state is saved, so we'd have to be careful not to remove the default repository information when managing the snapshot settings or always incur a state change when no settings are configured and the cluster has the default snapshot repository assigned.

Managing repository settings in ECE

When a Deployment is created in ECE, managing the snapshot settings is more straightforward than in SaaS, however it should be tested there too.

[marclop]

This won't be managed by the terraform provider, but by Elasticsearch settings themselves.