Advanced Search: Create new rake task with integration details

This issue is inspired by this slack thread (internal link)

We should create a new rake task (suggested name gitlab:advancedsearch:info so the name isn't tied to Elasticsearch) in order to help support engineers and self-managed admins. It should output all the details related to the Advanced Search integration.

We might find some inspiration from the gitlab:env:info rake task regarding the output style.

Here is a draft list of features:

• Search server distribution and version
• Output all gitlab indices, aliases, number of shards, replicas, and document count. Maybe even the size of the index.
• Main Advanced Search settings: Elasticsearch indexing, Search with Elasticsearch enabled, and Pause Elasticsearch indexing, Maximum file size indexed (KiB), and so on.
• Current size of initial and incremental queue
• Indexing status
• Number of pending migrations and their names
• Current active migration and its state
• Warning if there's a halted migration
• Warning if some indexes have write disabled (index.blocks.write to true)
• Warning if some indexes are set to read-only (index.blocks.read_only or index.blocks.read_only_allow_delete to true).
• Warning if some indexes have read disabled (index.blocks.read set to true)
• Warning if some indexes have index metadata reads and writes disabled (index.blocks.metadataset to true)

It should gracefully handle exceptions. For example, it shouldn't fail if the search server is down.

@vstoianovici @JohnMcGuire @changzhengliu @terrichu WDYT?