Add Readyset.route

[ethan-readyset]

Brief Description of Feature:
We should add a Readyset.route method that takes a block and routes to ReadySet any queries occurring within the block.

Background:
Our gem is designed around the expectation that ReadySet will be deployed as a read replica of sorts in the context of a Rails application. This adds some friction for developers because they need only route certain queries to ReadySet. To ease the burden of this, we want to provide an easy way for users to annotate that specific queries should be routed to ReadySet.

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Optional information to add

Proposed Solution:
Readyset.route should wrap a call to ActiveRecord::Base.connected_to with the shard keyword argument set to the name of the ReadySet database as configured in the user's config/database.yml. By default, the shard name will be :readyset, with an option for users to configure a name manually via our gem's configuration interface.

[ethan-readyset]

Closed in #47