chore(grpc-gateway) import grpc-gateway plugin into kong

As a result of an effort to improve external plugins maintainership,
we are importing currently-bundled plugins into Kong's main tree.
This plugin used to live in https://github.com/Kong/kong-plugin-grpc-gateway.

kong/plugins/grpc-gateway/CHANGELOG.md

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+# Table of Contents
+
+- [0.1.3](#013---20210603)
+- [0.1.2](#012---20201105)
+- [0.1.1](#011---20200526)
+- [0.1.0](#010---20200521)
+
+##  [0.1.3] - 2021/06/03
+
+- Fix typo from gatewat to gateway (#16)
+- Correctly clear URI args in rewrite (#23)
+- Map grpc-status to HTTP status code (#25)
+
+##  [0.1.2] - 2020/11/05
+
+- Allows `include` directives in protoc files by adding the
+main protoc file's directory as base for non-absolute paths
+- Clear up output buffer when getting partial response (#12)
+
+##  [0.1.1] - 2020/05/26
+
+- Set priority to 998 to avoid clash with other plugins.
+- Pin lua-protobuf to 0.3
+
+##  [0.1.0] - 2020/05/21
+
+- Initial release of gRPC gateway plugin for Kong.
+
+[0.1.3]: https://github.com/Kong/kong-plugin-grpc-gateway/compare/0.1.2...0.1.3
+[0.1.2]: https://github.com/Kong/kong-plugin-grpc-gateway/compare/0.1.1...0.1.2
+[0.1.1]: https://github.com/Kong/kong-plugin-grpc-gateway/compare/0.1.0...0.1.1
+[0.1.0]

kong/plugins/grpc-gateway/README.md

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+# Kong gRPC-gateway plugin
+
+A [Kong] plugin to allow access to a gRPC service via HTTP REST requests and translate requests and
+responses in a JSON format. Similar to
+[gRPC-gateway](https://github.com/grpc-ecosystem/grpc-gateway).
+
+## Description
+
+This plugin translates requests and responses between gRPC and HTTP REST.
+
+## Usage
+
+This plugin is intended to be used in a Kong route between a gRPC service and an HTTP endpoint.
+
+Sample configuration via declarative (YAML):
+
+```yaml
+_format_version: "1.1"
+services:
+- protocol: grpc
+  host: localhost
+  port: 9000
+  routes:
+  - protocols:
+    - http
+    paths:
+    - /
+    plugins:
+    - name: grpc-gateway
+      config:
+        proto: path/to/hello.proto
+```
+
+Same thing via the administation API:
+
+```bash
+$ # add the gRPC service
+$ curl -XPOST localhost:8001/services \
+  --data name=grpc \
+  --data protocol=grpc \
+  --data host=localhost \
+  --data port=9000
+
+$ # add an http route
+$ curl -XPOST localhost:8001/services/grpc/routes \
+  --data protocols=http \
+  --data name=web-service \
+  --data paths[]=/
+
+$ # add the plugin to the route
+$ curl -XPOST localhost:8001/routes/web-service/plugins \
+  --data name=grpc-gateway
+```
+
+The proto file must contain the
+[HTTP REST to gRPC mapping rule](https://github.com/googleapis/googleapis/blob/fc37c47e70b83c1cc5cc1616c9a307c4303fe789/google/api/http.proto).
+
+In the example we use the following mapping (note the `option (google.api.http) = {}` section):
+
+```protobuf
+syntax = "proto2";
+
+package hello;
+
+service HelloService {
+  rpc SayHello(HelloRequest) returns (HelloResponse) {
+    option (google.api.http) = {
+      get: "/v1/messages/{greeting}"
+      additional_bindings {
+        get: "/v1/messages/legacy/{greeting=**}"
+      }
+      post: "/v1/messages/"
+      body: "*"
+    }
+  }
+}
+
+
+// The request message containing the user's name.
+message HelloRequest {
+  string greeting = 1;
+}
+
+// The response message containing the greetings
+message HelloReply {
+  string message = 1;
+}
+```
+
+In this example, we can send following requests to Kong that translates to corresponding gRPC requests:
+
+```shell
+# grpc-go/examples/features/reflection/server $ go run main.go &
+
+curl -XGET localhost:8000/v1/messages/Kong2.0
+{"message":"Hello Kong2.0"}
+
+curl -XGET localhost:8000/v1/messages/legacy/Kong2.0
+{"message":"Hello Kong2.0"}
+
+curl -XGET localhost:8000/v1/messages/legacy/Kong2.0/more/paths
+{"message":"Hello Kong2.0\/more\/paths"}
+
+curl -XPOST localhost:8000/v1/messages/Kong2.0 -d '{"greeting":"kong2.0"}'
+{"message":"Hello kong2.0"}
+```
+
+All syntax defined in [Path template syntax](https://github.com/googleapis/googleapis/blob/fc37c47e70b83c1cc5cc1616c9a307c4303fe789/google/api/http.proto#L225)
+is supported.
+
+Currently only unary requests are supported, streaming requests are not supported.
+
+## Dependencies
+
+The gRPC-gateway plugin depends on [lua-protobuf], [lua-cjson] and [lua-pack]
+
+[Kong]: https://konghq.com
+[lua-protobuf]: https://github.com/starwing/lua-protobuf
+[lua-cjson]: https://github.com/openresty/lua-cjson
+[lua-pack]: https://github.com/Kong/lua-pack
+