Add toggle to allow empty Content-Length headers

In go 1.22, they are changing the default behavior in the `net/http`
standard library to **REJECT** requests and responses containing an
invalid, empty `Content-Length` header.
Source: https://tip.golang.org/doc/go1.22#minor_library_changes

As per the authors,
> The previous behavior may be restored by setting GODEBUG field httplaxcontentlength=1.

This commit enables operators to set this GODEBUG field by configuring
`gorouter`'s `go.httplaxcontentlength` property.

By default, this property will be disabled and gorouter will begin to
reject requests that contain invlaid, empty `Content-Length` headers.

[#187125229](https://www.pivotaltracker.com/story/show/187125229)

jobs/gorouter/spec

@@ -599,6 +599,14 @@ properties:
             street_address: []
             postal_code: []
     default: []
+
   healthchecker.failure_counter_file:
     description: "File used by the healthchecker to monitor consecutive failures."
     default: /var/vcap/data/gorouter/counters/consecutive_healthchecker_failures.count
+
+  go.httplaxcontentlength:
+    description: |
+        Environment Flag to temporarily allow requests containing an invalid, empty `Content-Length` header for backwards compatibility.
+        This toggle allows operators to add the `GODEBUG` field `httplaxcontentlength=1`, as allowable per the [go 1.22 release documentation](https://tip.golang.org/doc/go1.22#minor_library_changes).
+        Defaults to `false` as the default behavior in go 1.22+ is to reject these requests.
+    default: false

jobs/gorouter/templates/bpm.yml.erb

@@ -26,5 +26,9 @@ bpm = {
   ]
 }
 
+if p('go.httplaxcontentlength') == true
+      bpm['processes'][0]['env']['GODEBUG'] << ',httplaxcontentlength=1'
+end
+
 YAML.dump(bpm)
 %>

spec/gorouter_templates_spec.rb

@@ -1628,6 +1628,31 @@
       end
     end
   end
+
+  describe 'bpm' do
+    let(:deployment_manifest_fragment) { {} }
+    let(:template) { job.template('config/bpm.yml') }
+    let(:rendered_template) { template.render(deployment_manifest_fragment) }
+    subject(:parsed_yaml) { YAML.safe_load(rendered_template) }
+
+    context 'GODEBUG' do
+      it 'defaults to netdns=cgo' do
+        expect(parsed_yaml['processes'][0]['env']['GODEBUG']).to eq('netdns=cgo')
+      end
+
+      context 'when go.HTTPLaxContentLength is enabled' do
+        before do
+          deployment_manifest_fragment['go'] = {
+            'httplaxcontentlength' => true,
+          }
+        end
+
+        it 'sets httplaxcontentlength=1 correctly' do
+          expect(parsed_yaml['processes'][0]['env']['GODEBUG']).to eq('netdns=cgo,httplaxcontentlength=1')
+        end
+      end
+    end
+  end
 end
 
 # rubocop:enable Layout/LineLength