Update pom.xml to use another 2.5.0 protoc as external protobuf

Currently, there is no protoc 2.5.0 for release [1]. So we can make a new one for ARM specific. For make sure that could work on ARM.

We will introduce a new ARM artifact for protoc, group_id is org.openlabtesting.protobuf .. This is just used for protobuf-maven-plugin to compile .proto files. As the 3.X version of protoc support ARM already. So this won't affect the internal protoc usage, which is 3.5.1-1 now.

[1]https://github.com/protocolbuffers/protobuf/issues/3844#issuecomment-343355946