Use Bazel to build the entire backend and perform API code generation (…

…#609)

* Use Bazel to build the entire backend.

This also uses Bazel to generate code from the API definition in the
proto files.

The Makefile is replaced with a script that uses Bazel to first generate
the code, and then copy them back into the source tree.

Most of the BUILD files were generated automatically using Gazelle.

* Fix indentation in generate_api.sh

* Clean up WORKSPACE

* Add README for building/testing backend.

Also fix the missing licenses in the generated proto files.

* Add license to files under go_http_client

.gitignore

@@ -43,4 +43,7 @@ ml.iml
 *.egg-info
 
 # go vendor
-vendor
+vendor
+
+# Bazel output artifacts
+bazel-*

BUILD.bazel

@@ -0,0 +1,7 @@
+load("@bazel_gazelle//:def.bzl", "gazelle")
+
+# gazelle:prefix github.com/kubeflow/pipelines
+# gazelle:resolve proto protoc-gen-swagger/options/annotations.proto @com_github_grpc_ecosystem_grpc_gateway//protoc-gen-swagger/options:options_proto
+# gazelle:resolve proto go protoc-gen-swagger/options/annotations.proto @com_github_grpc_ecosystem_grpc_gateway//protoc-gen-swagger/options:go_default_library
+# gazelle:resolve go github.com/kubeflow/pipelines/backend/api/go_client //backend/api:go_default_library
+gazelle(name = "gazelle")