Changes:
1. Replace github.com/grpc-ecosystem/grpc-gateway with github.com/grpc-ecosystem/grpc-gateway/v2
   in all golang source files and in scripts/genproto.sh.
2. The only exception is to manually replace github.com/grpc-ecosystem/grpc-gateway/protoc-gen-swagger
   with github.com/grpc-ecosystem/grpc-gateway/v2/protoc-gen-openapiv2.

Signed-off-by: Benjamin Wang <[email protected]>

Firstly run the following commands under `api` and `tools/mod` directories respectively:
1. go get github.com/grpc-ecosystem/grpc-gateway/[email protected]
2. go mod tidy

Secondly run command below,
  ./scripts/fix.sh

Signed-off-by: Benjamin Wang <[email protected]>

The third_party/googleapis has been removed from github.com/grpc-ecosystem/grpc-gateway/v2,
and the solution for now is to get them included in etcd repo directly. Please refer to
grpc-ecosystem/grpc-gateway#1065 (comment)

In the future, we may consider to manage the protobuf dependencies using
https://github.com/bufbuild/buf. Please see
grpc-ecosystem/grpc-gateway#1065 (comment).

Signed-off-by: Benjamin Wang <[email protected]>

…o on command line

The value of `go_package` in google/protobuf/descriptor.proto is "descriptor",
which isn't supported by protoc-gen-grpc-gateway v2. The error message is:
```
stderr: protoc-gen-grpc-gateway: invalid Go import path "descriptor" for "google/protobuf/descriptor.proto"
stderr:
stderr: The import path must contain at least one period ('.') or forward slash ('/') character.
```

The solution is to specify the go import path on command line in format: M${PROTO_FILE}=${GO_IMPORT_PATH}.

Signed-off-by: Benjamin Wang <[email protected]>