CBAPI-5227: FileCredentialProvider - Support UTF-16 Encoding

[abowersox-cb]

Pull request checklist

Please check if your PR fulfills the following requirements:

• [x] Docs have been reviewed and added / updated if needed (for bug fixes / features)
• [x] Tests have been added that prove the fix is effective or that the feature works.
• [x] New and existing tests pass locally with the changes.
• [x] Code follows the style guidelines of this project (PEP8, clean code, linter).
• [x] A self-review of the code has been done.

What is the ticket or issue number?

https://jira.carbonblack.local/browse/CBAPI-5227

Pull Request Description

Added the ability for the FileCredentialProvider to parse UTF-16 configuration files in addition to UTF-8 ones. UTF-16 files must begin with a Unicode BOM character, U+FEFF. UTF-8 files may begin with an encoded BOM character, but need not.

Does this introduce a breaking change?

• [ ] Yes
• [x] No

How Has This Been Tested?

New tests have been added (with appropriate test files in the repository) that test the added file formats. Coverage in file_credential_provider.py stands at 100%.