Stream Tag Builder: Support :renderable arguments

When passed a valid `:renderable` option (like an object that responds
to `#render_in`), treat that object as both the `<turbo-stream>`
element's template contents _and_ attempt to treat it as the target.

For example, consider a simplified "component" class:

```ruby
class Component
  extend ActiveModel::Naming

  def initialize(id:, content:) = (@id, @content = id, content)
  def render_in(...) = @content
  def to_key = [@id]
end

component = Component.new(id: 1, content: "Hello, world")

turbo_stream.update(component) # => <turbo-stream action="update" target="component_1"><template>Hello, world</template></turbo-stream>
```

app/models/turbo/streams/tag_builder.rb

@@ -271,6 +271,8 @@ def action_all(name, targets, content = nil, allow_inferred_rendering: true, **r
   private
     def render_template(target, content = nil, allow_inferred_rendering: true, **rendering, &block)
       case
+      when target.respond_to?(:render_in) && content.nil?
+        target.render_in(@view_context, &block)
       when content.respond_to?(:render_in)
         content.render_in(@view_context, &block)
       when content

test/streams/streams_helper_test.rb

@@ -3,8 +3,32 @@
 class TestChannel < ApplicationCable::Channel; end
 
 class Turbo::StreamsHelperTest < ActionView::TestCase
+  class Component
+    extend ActiveModel::Naming
+
+    def initialize(id:, content:) = (@id, @content = id, content)
+    def render_in(...) = @content
+    def to_key = [@id]
+  end
+
   attr_accessor :formats
 
+  test "supports valid :renderable option object with nil content" do
+    component = Component.new(id: 1, content: "Hello, world")
+
+    assert_dom_equal <<~HTML.strip, turbo_stream.update(component)
+      <turbo-stream action="update" target="streams_helper_test_component_1"><template>Hello, world</template></turbo-stream>
+    HTML
+  end
+
+  test "supports valid :renderable option object with content" do
+    component = Component.new(id: 1, content: "Hello, world")
+
+    assert_dom_equal <<~HTML.strip, turbo_stream.update(component, "Raw content")
+      <turbo-stream action="update" target="streams_helper_test_component_1"><template>Raw content</template></turbo-stream>
+    HTML
+  end
+
   test "with streamable" do
     assert_dom_equal \
       %(<turbo-cable-stream-source channel="Turbo::StreamsChannel" signed-stream-name="#{Turbo::StreamsChannel.signed_stream_name("messages")}"></turbo-cable-stream-source>),
@@ -43,4 +67,12 @@ class Turbo::StreamsHelperTest < ActionView::TestCase
       <turbo-stream action="highlight" targets=".a-selector"><template></template></turbo-stream>
     HTML
   end
+
+  test "supports valid :partial option objects" do
+    message = Message.new(id: 1, content: "Hello, world")
+
+    assert_dom_equal <<~HTML.strip, turbo_stream.update(message)
+      <turbo-stream action="update" target="message_1"><template><p>Hello, world</p></template></turbo-stream>
+    HTML
+  end
 end