fix: support specifying custom sts endpoint (#23) * Support specifying custom sts endpoint Signed-off-by: Bogdan Kyryliuk <bogdankyryliuk@dropbox.com> * bump version Signed-off-by: Bogdan Kyryliuk <bogdankyryliuk@dropbox.com> Co-authored-by: Bogdan Kyryliuk <bogdankyryliuk@dropbox.com>