Inconsistencies across functions that retrieve auth tokens

[dpprdan]

There are some inconsistencies across the functions that retrieve auth tokens from env vars/options w.r.t. documentation and whether or not they are exported:

function	roxygen docs	@noRd	@keywords internal	@export	quiet argument
github_pat()	:white_check_mark:	:white_check_mark:	:white_check_mark:	:x:	:white_check_mark:
gitlab_pat()	:white_check_mark:	:x:	:white_check_mark:	:white_check_mark:	:white_check_mark:
bitbucket_user()	:x:	:x:	:x:	:x:	:white_check_mark:
bitbucket_password()	:x:	:x:	:x:	:x:	:white_check_mark:
git_credentials()	:white_check_mark:	:x:	:white_check_mark:	:white_check_mark:	:x:

"roxygen docs" means whether the function is documented in roxygen (title, description, @param), the other columns are probably self-explanatory.

Suggestion: make all functions behave consistently like gitlab_pat(). If you agree, I can issue a PR.

(There is probably a good reason for why git_credentials() uses getOption() instead of Sys.getenv(), so I would not change that).