Add support for Cloud Vision Product Search

[dpebot]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > Cloning googleapis.
synthtool > Running generator for google/cloud/vision/artman_vision_v1.yaml.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/php/google-cloud-vision-v1.
synthtool > Replaced 'Copyright \\d{4}' in src/V1/Gapic/ImageAnnotatorGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V1/Gapic/ProductSearchGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/System/V1/ImageAnnotatorSmokeTest.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Snippet/V1/ImageAnnotatorClientTest.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/V1/ProductSearchClientTest.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/V1/ImageAnnotatorClientExtensionTest.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/V1/ImageAnnotatorClientTest.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/WebDetection_WebEntity.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/TextAnnotation_DetectedBreak_BreakType.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/TextAnnotation_DetectedLanguage.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/ProductSearchResults_Result.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/OperationMetadata_State.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/WebDetection_WebLabel.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/FaceAnnotation_Landmark.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/Product_KeyValue.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/Feature_Type.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/TextAnnotation_DetectedBreak.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/WebDetection_WebImage.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/FaceAnnotation_Landmark_Type.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/WebDetection_WebPage.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/Block_BlockType.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/TextAnnotation_TextProperty.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/BatchOperationMetadata_State.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/ProductSearchResults_GroupedResult.php.
synthtool > Cleaned up 1 temporary directories.

[jdpedrie]

Added Google\Cloud\Vision\V1\ProductSearchClient (synthtool excludes these), and added it to the docs table of contents.

@tmatsuo @dwsupplee can one of you also confirm?