feat(tdesktop): support key_datas

[ernado]

There can be no key_data in latest tdesktop format

cc: @tdakkota

[ernado]

So while trying to load tdata:

Error: failed to read accounts:
    main.run
        /src/main.go:70
  - open key_data:
    github.com/gotd/td/session/tdesktop.ReadFS
        /go/pkg/mod/github.com/gotd/[email protected]/session/tdesktop/tdesktop.go:36
  - file "key_data" not found:
    github.com/gotd/td/session/tdesktop.open
        /go/pkg/mod/github.com/gotd/[email protected]/session/tdesktop/file.go:60

File tree:

tdata/
├── D877F783D5D3EF8C
│   └── maps
├── D877F783D5D3EF8Cs
└── key_datas