arrow

GH-38033: [R] Allow `code()` to return package name prefix. (#38144)

### Rationale for this change
#38033 

### What changes are included in this PR?
- ~~Added `get_pkg_ns()` helper.~~
- ~~Added `call_name` private method to `DataType` class to store the string name used in the code call. Refactored `code()` public method to use `call_name`.~~
- Converted all `$code() call(...)` to `$code(namespace = FALSE) call2(..., .ns = if(namespace) "arrow")` in `DataType`, `Schema`, and `DictionaryType`.
- Added `code` to `Schema` docstring.
- Updated `expect_code_roundtrip` to test roundtrip with and without namespace, and check match/no match for `arrow::` depending on namespace argument.

### Are these changes tested?
* All tests pass, including lintr checks.

### Are there any user-facing changes?
Yes, user-facing changes, but no breaking changes to any public APIs.
* Closes: #38033

Lead-authored-by: orgadish <48453207+orgadish@users.noreply.github.com>
Co-authored-by: Nic Crane <thisisnic@gmail.com>
Signed-off-by: Nic Crane <thisisnic@gmail.com>