(SERVER-2470) Draft implementation for list_all_transports

To test out, run `rake spec_prep` and use the following setup in pry:

```
require 'puppet'
require 'puppet/resource_api/transport'
require 'puppet/settings'

Puppet.initialize_settings
Puppet[:modulepath] = 'spec/fixtures/modules'

# env = Puppet.lookup(:environments).get!('production')
# files = Puppet::Util::Autoload.files_to_load('puppet/transport/schema', env)

Puppet::ResourceApi::Transport.list_all_transports('production')
```

Author: DavidS

lib/puppet/resource_api/transport.rb

@@ -20,11 +20,39 @@ def register(schema)
   module_function :register # rubocop:disable Style/AccessModifierDeclarations
 
   # retrieve a Hash of transport schemas, keyed by their name.
+  # Only already loaded transports are returned.
   def list
     Marshal.load(Marshal.dump(transports))
   end
   module_function :list # rubocop:disable Style/AccessModifierDeclarations
 
+  # retrieve a Hash of transport schemas, keyed by their name.
+  # This uses the Puppet autoloader, so beware of your setup.
+  # @api private
+  def list_all_transports(force_environment = nil)
+    if force_environment.nil?
+      load_all_schemas
+      Marshal.load(Marshal.dump(transports))
+    else
+      env = Puppet.lookup(:environments).get!(force_environment)
+      Puppet.override({ current_environment: env }, 'current env for list_all_transports') do
+        load_all_schemas
+        Marshal.load(Marshal.dump(transports))
+      end
+    end
+  end
+  module_function :list_all_transports # rubocop:disable Style/AccessModifierDeclarations
+
+  # Loads all schemas using the Puppet Autoloader.
+  def self.load_all_schemas
+    require 'puppet'
+    require 'puppet/settings'
+    require 'puppet/util/autoload'
+    autoloader = Puppet::Util::Autoload.new(self, 'puppet/transport/schema')
+    autoloader.loadall(Puppet.lookup(:current_environment))
+  end
+  private_class_method :load_all_schemas
+
   def connect(name, connection_info)
     validate(name, connection_info)
     require "puppet/transport/#{name}"