Add Apisearch format file

[mmoreram]

Following the Apisearch format specification (not in documentation yet, ping @mmoreram), this plugin should create a new file containing the whole index in CSV. We should add an encrypted path in order to keep this file private and not expose it to everyone (like .../plugin/apisearch/ds7d89s78d7s89d7s89d7s.feed.as.

After that, we will have the possibility to process this file once each 24 hours (or variable).