Implement Pass and Dialect plugins for mlir-opt

Implementation of Pass and Dialect Plugins that mirrors LLVM Pass Plugin  implementation from the new pass manager.

Currently the implementation only supports using the pass-pipeline option for adding passes. This restriction is imposed by the `PassPipelineCLParser` variable in mlir/lib/Tools/mlir-opt/MlirOptMain.cpp:114 that loads the parse options statically before parsing the cmd line args.

```
mlir-opt stanalone-plugin.mlir --load-dialect-plugin=lib/libStandalonePlugin.so --pass-pipeline="builtin.module(standalone-switch-bar-foo)"
```

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D147053