[Variant] Implement `VariantObject::field` and `VariantObject::fields`

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
I was trying to write some tests for the VariantBuilder from @PinkCrow007 in

• Add documentation and examples to VariantBuilder PinkCrow007/arrow-rs#1

However, I found that VariantObject::field was not implemented: https://github.com/apache/arrow-rs/blob/71ee9d9aa090a4d733c8404c6edb8d34d767a0c4/parquet-variant/src/variant.rs#L317-L316

So when I wrote code to access a field from an object

assert_eq!(variant_object.field("first_name"), Variant::from("Jiaying")

It panic'd on me

Describe the solution you'd like
Implement the two methods named in this ticket. The code is here:

arrow-rs/parquet-variant/src/variant.rs

Lines 311 to 317 in 2f2e705

	pub fn fields(&self) -> Result<impl Iterator<Item = (&'m str, Variant<'m, 'v>)>, ArrowError> {
	todo!();
	#[allow(unreachable_code)] // Just to infer the return type
	Ok(vec![].into_iter())
	}
	pub fn field(&self, _name: &'m str) -> Result<Variant<'m, 'v>, ArrowError> {
	todo!()

Describe alternatives you've considered

1. Implement the code
2. Update the existing tests here:

   arrow-rs/parquet-variant/tests/variant_interop.rs

   Lines 95 to 99 in 2f2e705

   	"object_primitive" => {
   	assert!(matches!(variant, Variant::Object(_)));
   	assert_eq!(metadata.dictionary_size(), 7);
   	let dict_val = metadata.get_field_by(0)?;
   	assert_eq!(dict_val, "int_field");

To retrieve each field and verify its value

The expected answers are here:

https://github.com/apache/parquet-testing/blob/84d525a8731cec345852fb4ea2e7c581fbf2ef29/variant/data_dictionary.json#L46-L54

Additional context

[alamb]

FYI @mkarbo

[alamb]

I think this is a good first issue for anyone who wants to learn about variant as it is well specified and has well known results

[KrishnaSindhur]

take

[scovich]

Oh! @KrishnaSindhur -- I already had a PR almost ready to go and just saw this issue...

[alamb]

label_issue.py automatically added labels {'parquet'} from #7666