(FM-7839) Implement 'to_json' method for ResourceShim for bolt

bolt's `get_resources` uses the `libexec/query_resources.rb` to retrieve
resources as json from the target nodes. Especially https://github.com/puppetlabs/bolt/blob/7dec6e49d66c7f9ae06088d46ee3330de0ed2fed/libexec/query_resources.rb#L71
causes `ResourceShims` to fail as they did not implement `to_json`.

This integration test models what `libexec/query_resources.rb` is doing.
The added `to_json(*)` method now passes this test, and - shown in manual
testing - works for bolt.

Author: DavidS

lib/puppet/resource_api/glue.rb

@@ -45,6 +45,13 @@ def to_hash
       values
     end
 
+    def to_json(*)
+      attrs = filtered_keys.map { |k| [k.to_s, values[k]] unless values[k].nil? }
+      attributes = Hash[*attrs.compact.flatten]
+      resource = { title => attributes }
+      resource.to_json
+    end
+
     # attribute names that are not title or namevars
     def filtered_keys
       values.keys.reject { |k| k == :title || !attr_def[k] || (attr_def[k][:behaviour] == :namevar && @namevars.size == 1) }

spec/integration/resource_api_spec.rb

@@ -223,6 +223,10 @@ def get(_context)
         describe 'its title' do
           it { expect(instance.to_resource.title).to eq 'somename' }
         end
+
+        it 'can serialize to json' do
+          expect({ 'resources' => [instance.to_resource] }.to_json).to eq '{"resources":[{"somename":{"ensure":"absent","boolean_param":false,"integer_param":99,"float_param":3.21,"ensure_param":"present","variant_pattern_param":"1234ABCD","url_param":"http://www.puppet.com","string_array_param":"d","e":"f","string_param":"default value"}}]}' # rubocop:disable Metrics/LineLength
+        end
       end
 
       it('ensure is reported as a symbol') { expect(instance[:ensure]).to be_a Symbol }

spec/puppet/resource_api/glue_spec.rb

@@ -47,6 +47,23 @@
       end
     end
 
+    describe '.to_json' do
+      it { expect(instance.to_json).to eq '{"title":{"attr":"value","attr_ro":"fixed"}}' }
+
+      context 'with nil values' do
+        subject(:instance) do
+          described_class.new({ namevarname: title, attr: nil, attr_ro: 'fixed' }, 'typename', [:namevarname],
+                              namevarname: { type: 'String', behaviour: :namevar, desc: 'the title' },
+                              attr: { type: 'String', desc: 'a string parameter' },
+                              attr_ro: { type: 'String', desc: 'a string readonly', behaviour: :read_only })
+        end
+
+        it 'doesn\'t output them' do
+          expect(instance.to_json).to eq '{"title":{"attr_ro":"fixed"}}'
+        end
+      end
+    end
+
     describe '.to_hierayaml' do
       it { expect(instance.to_hierayaml).to eq "  title:\n    attr: value\n    attr_ro: fixed\n" }