Implement ActiveRecord integration configuration

[delner]

Following from #450 , this pull request implements the integration configuration core for ActiveRecord, and leverages its multiplexing feature to implement the API for multi-database configuration (based on #404), permitting multiple databases to be configured like so:

Datadog.configure do |c|
  # Where 'gadget_db' is a key in the `database.yml` configuration
   c.use :active_record, describes: :gadget_db do |gadget_db|
    gadget_db.tracer = tracer
    gadget_db.service_name = gadget_db_service_name
  end

  # You can also provide connection strings as a key
  c.use :active_record, describes: 'mysql2://root@127.0.0.1:3306/mysql' do |backup_db|
    backup_db.service_name = 'backup-db'
  end
end

[pawelchcki]

WDYT about putting this file in a folder named vendor? Similarily how gems are put in ROOT/vendor but here it would be something like:

lib/ddtrace/contrib/active_record/vendor/configuration/connection_specification.rb

Its a small and cosmetic change, but it would make me feel much better about including files from different projects verbatim.

We could actually insert this file without any modifications too, and add small note in the vendor folder, and exclude the whole prefix in the Rubocop.

[delner]

Hmmm, I like that idea. Would make the Rubocop and other stuff much simpler to deal with too. I'll try this.