Add option for generating `toString()` methods basing on fields

[alexander-yevsyukov]

Have a look at the Twitter thread which hinted me on this feature. The thread shows the way TypeScript defines new types basing on values of other types used in a string form.

Currently we have an ability in Java to add a mix-in interface which would allow to have a custom toString(), but it's too much work. A more simple approach would be to have a type option, which would be used by the CoreJvm Compiler for Java.

Obviously, we'll need to add the option to spine/options.proto under Base Libraries repo.

Template-based toString()

The option declaration could look something like this:

message Length {
   option (string_form).template = "${value}${units}";
   int value = 1 [(required) = true];
   LengthUnit units = 2 [(required) = true];
}

Delegated string form

Similarly to how we arrange calculated validation using MessageValidator SPI, we can have generated toString() delegating to a Stringifier of the corresponding message class. This behaviour would have a trigger in the message declaration via the delegated field:

message Length {
   option (string_form).delegated = true;
   int value = 1 [(required) = true];
   LengthUnit units = 2 [(required) = true];
}

The name delegated hides the implementation details. The generated code for languages other than Java may want to use other approaches that won't have the Stringifier concept.