chore(samples): add samples for custom infotype rules (#76)

Add missing python samples for https://cloud.google.com/dlp/docs/creating-custom-infotypes-rules#dlp_inspect_string_without_overlap-java

- [x] Make sure to open an issue as a [bug/issue](https://github.com/googleapis/python-dlp/issues/new/choose) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
- [x] Ensure the tests and linter pass
  - Some tests in other files are already broken, but `pytest custom_infotype_test.py` succeeds. 
- [x] Code coverage does not decrease (if any source code was changed)
- [x] Appropriate docs were updated (if necessary)



Fixes internal bugs:
b/156968601
b/156970772
b/156970547
b/156969966
b/156970576
b/156974339
b/156969968

Author: sushicw