add github.com/99designs/keyring for macOs provider

[kaplanelad]

Following #158 pr, adding github.com/99designs/keyring vendor package for @vjeantet to implement macOs provider.

@vjeantet if you seeing in your PR that you need more vendor pacakge please let me know and i'll push them into this branch.

You can fork from this branch and adding the provider implementation. Thanks for the understanding!

[vjeantet]

PR created here : https://github.com/tellerops/teller/pull/166

[jondot]

closing for inactivity