Generic Value and SliceValue implementation

[mmatczuk]

Using Go generics it's possible to greatly streamline the process of binding custom value types.
I have created a library as a POC github.com/mmatczuk/anyflag.
To work with it users have to provide the type and a parser (aka fromString) function.

Example:

    var ba *url.Userinfo
    cmd.Flags().VarP(anyflag.NewValue[*url.Userinfo](nil, &ba, parseBasicAuth), "basic-auth", "", "basic auth")

    var bas []*url.Userinfo
    cmd.Flags().VarP(anyflag.NewSliceValue[*url.Userinfo](nil, &bas, parseBasicAuth), "basic-auth", "", "basic auth")

See full running example in example_test.go.

It's also possible to replace the concrete implementations here with the generic one largely reducing the code surface.

Please let me know what do you think.