[C++] Support iterable types in RowEncodeTrait

[PragmaTwice]

What do these changes do?

In this PR we support any iterable types for RowEncodeTrait and its combination with struct types, e.g.

std::vector<int>

// nested array type
std::vector<std::vector<int>>

// class type in array type
struct A { ... }
std::vector<A>

// array in class type
struct B {
  std::vector<...> field1;
  ...
}

Check code requirements

• tests added / passed (if needed)
• Ensure all linting tests pass, see here for how to run them

[chaokunyang]

LGTM. one more thing, do we need to add an ArrayEncoder in array_encoder.h ?

[chaokunyang]

Those meta programing codegen is really impressive, learned a lot from your code 😄

[PragmaTwice]

do we need to add an ArrayEncoder in array_encoder.h ?

I plan to extend RowEncoder<T> rather than adding a new class, so users don't need to use different class to encode different types.

[chaokunyang]

do we need to add an ArrayEncoder in array_encoder.h ?

I plan to extend RowEncoder<T> rather than adding a new class, so users don't need to use different class to encode different types.

OK, got it