Use ServerHooks to validate request message content

[manhdaovan]

If the RequestReceived of ServerHooks struct is changed to func(ctx context.Context, req interface{}) (context.Context, error) instead of func(context.Context) (context.Context, error), go-proto-validators can be integrated to validate request message content.

For example:

type Validator interface{
    Validate() error
}
func ValidateReq(ctx context.Context, req interface{}) (context.Context, error){
    if r, ok := req.(Validator); ok {
        if err := r.Validate(); err != nil {
            return nil, err
        }
    }

    // other logic
}

[spenczar]

This has come up a few times, and after some thorough discussion, we've decided not to do it. Previous issues have the discussion: https://github.com/twitchtv/twirp/issues/187 https://github.com/twitchtv/twirp/issues/184