Use typed arrays for fields

[rcorre]

Given:

message Foo {
    repeated int32 i = 1;
    repeated string s = 2;
    repeated Bar b = 3;
}

Currently we generate:

get_i() -> Array
get_s() -> Array
get_b() -> Array

With gdscript warnings enabled, this tends to generate lots of warnings in consuming code, and require "unsafe" casts that must be ignored.
It would be nice if we could generate typed return values, like:

get_i() -> Array[int]
get_s() -> Array[string]
get_b() -> Array[Bar]