[Elasticsearch] schema versions table

A continuation of https://gitlab.com/gitlab-org/gitlab-ee/issues/328

After converting ActiveModel related search modules in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14428, we need to create database table to store a list of schema versions. A rough proposal would be the following:

• friendly name
• index_name (e.g. "gitlab-development-V12p1")
• url
• aws region
• aws access key
• aws secret key
• version (e.g. 'V12p1')
• number of shards
• number of replicas
• read (boolean, and only one row can be true)

Then we modify Elastic::ProxyUtil to initiate proxies from these settings.