Extend vttest.TopoData to implement `pflag.Value`, and make functions implicit (#10994)
Extend vttest.TopoData to implement `pflag.Value`, and make functions implicit (#10994)

Before, these helper functions explicitly returned the `flag.Value` type,
making it impossible to use with `pflag`. Attempting to do so would
produce compiler errors like:

```
cannot use vttest.JSONTopoData(&tpb) (value of type "flag".Value) as pflag.Value value in argument to pflag.Var: "flag".Value does not implement pflag.Value (missing method Type)
```

Changing the return type to the struct, which _implicitly_ implements both
`flag.Value` and `pflag.Value` allows us to use these function in both old
(`flag`) and new (`pflag`) Var functions.

Closes #10988.

Signed-off-by: Andrew Mason <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@planetscale.com>

Signed-off-by: Andrew Mason <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@planetscale.com>
mahak/vitess,mahak/vitess,mahak/vitess,vitessio/vitess,vitessio/vitess,vitessio/vitess,mahak/vitess,vitessio/vitess,mahak/vitess,vitessio/vitess,mahak/vitess,mahak/vitess,vitessio/vitess,vitessio/vitess,mahak/vitess,vitessio/vitess