ENH: IO support for R data files with C extension
- [X] follows up on #40884
- [X] tests added / passed
- [X] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [X] whatsnew entry
- [X] user_guide/io entry




### Proposed RData I/O module interfaces to the C library: [librdata](https://github.com/WizardMac/librdata). 

Overall, this PR includes following changes:
- **setup.py**: pandas/setup.py (new rdata section at the bottom)
- **librdata**: pandas/_libs/src/librdata (C and header files and iconv scripts)
- **rdata IO**: pandas/io/rdata (Cython and Python scripts)
- **frame.py**: pandas/core/frame.py (for `DataFrame.to_rdata`)
- **tests**: pandas/tests/io/test_rdata.py
- **tests data**: pandas/tests/io/data/rdata (R data files in gzip compression)
- **docs**: librdata license, user_guide/io.rst, whatsnew/v1.3.0.rst

Note: special handling of `iconv`, a system resource built-in to Unix machines, is required:
- For Linux, to centralize from different locations, the `iconv.h` header of the GNU C library is included.
- For Mac, setup.py points to system folders, `/usr/include` and `/usr/lib`, which may differ from users installs.
- For Windows , since `iconv` is not built-in, two counterpart files (.h and .c script) were added from this repo, [win-iconv](https://github.com/win-iconv/win-iconv), where its readme indicates code is placed in the public domain.