Pretty Print Tree in ORAS Discover

[shizhMSFT]

Before:

$ oras discover --plain-http --output tree $reg/test:base
localhost:5000/test:base
├── [foo]sha256:3c6ed9d37e825a80565d2e9602d727cd076ed7c5bf15c9193f6df6248735828c
├── [foo]sha256:4cbfbbe2e37038d8b6bffe99622c92709d9703e95f85851fc08953de176951a1
└── [bar]sha256:ccb9446f7dc9a55448c7f17dea6eda2c74ceda1c021dc747737c6f7b05584b0f

After:

$ oras discover --plain-http --output tree $reg/test:base
localhost:5000/test:base
├── foo
│   ├── sha256:3c6ed9d37e825a80565d2e9602d727cd076ed7c5bf15c9193f6df6248735828c
│   └── sha256:4cbfbbe2e37038d8b6bffe99622c92709d9703e95f85851fc08953de176951a1
└── bar
    └── sha256:ccb9446f7dc9a55448c7f17dea6eda2c74ceda1c021dc747737c6f7b05584b0f