ENH20521 Added metadata argument to DataFrame.to_parquet 
The argument allows for custom file metadata updating the default one.
Closes #20521 

Checklist for other PRs (remove this part if you are doing a PR for the pandas documentation sprint):

- [x] closes #xxxx
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Checklist from comments:

- [ ] add example in to_parquet docstring
- [ ] add API docs
- [ ] investigate fastparquet
- [ ] correct kwargs
- [ ] add test for warning
- [ ] address other comments
