feat: Add schema construction DSL

[bkietz]

Construct nanoarrow::UniqueSchemas with a tiny C++ DSL:

  schema{children{
      {"i", "i32",
       metadata{
           "some_key=some_value",
       }},
      {"i", "i32",
       dictionary{{"u"}},
       metadata{
           "some_key=some_value",
       },
       ARROW_FLAG_NULLABLE},
  }};

The python-equivalent signature is approximately:

def schema(format="+s", name="", metadata=[], flags=0, children=[], dictionary=None):

After the first two arguments, all the rest can be provided in any order.

[paleolimbot]

Just some initial thoughts! If we're really headed for readable schema construction, Arrow C++-like constructors (e.g., int32("field name")) are more likely to be what we want (and might be substantially less complicated to implement). I probably would have gone towards templating on ArrowType and using our C library constructors instead of introducing another way to create schemas.

It's worth looking at how https://github.com/man-group/sparrow does this (I think they have something pretty similar to construct types/schemas).

I was hoping to merge #561 before adding anything to the testing library (it could use a review!).

[paleolimbot]

Maybe factory?

[paleolimbot]

Would std::vector<std::pair<std::string, std::string>> work? I'm not sure the world is clamouring to create metadata keys that contain the = symbol but it seems like it would be nice to avoid string parsing if we can.

[paleolimbot]

In tests I think I would still prefer to see the C calls (but perhaps this is not where you were headed with this)