Add MySql support

[adlamas]

• Update adapter helper to support MySQL json types
• Update generator spec to permit using MySql
• Update migration spec
• Update CI to check MySql's support

[guillermoap]

Requesting changes mainly because of the missing spec. Looking great 💪🏻

[guillermoap]

can we use case? I feel it's cleaner that returns

[adlamas]

This is the way it would be with case

    def self.uuid_type
      case adapter
      when 'postgres'
        'uuid'
      when 'mysql2'
        'string'
      else
        'string'
      end
    end

    def self.json_type
      case adapter
      when 'postgres'
        'jsonb'
      when 'mysql2'
        'json'
      else
        'string'
      end
    end

    def self.adapter
      ActiveRecord::Base.connection.adapter_name.downcase
    end
  end

Personally, I prefer it with if statements, I don't see it more clear

[guillermoap]

Also remember to bump version, and the run a bundle install