Consider boundaries set by other fields

[dmdashenkov]

Right now, we only allow constant boundaries for number fields, e.g. (max).value = "42".

We could consider making the boundary values dynamic. For example:

message LocalDate {
    int32 year = 1;

    Month month = 2;

    uint32 day = 3 [(max).expression = "month->max_days"];
}

Here, max_days would be an option defined on every value of the Month enum.

[armiol]

I would use a syntax compatible with the one of FieldMasks:

          uint32 day = 3 [(max).expression = "month.max_days"];