Add SlackTokens option for slack credentials

[trufflesteeeve]

Is there a way to do this without the tokens name, and instead having just bot_token and app_token be the credential? I don't think so but I am less familiar with protobufs.

[dustin-decker]

It's needed to ensure that we can differentiate on credential types.
It is possible to include it as top level fields, but then you lose out on having different credential types and need to have application logic sift through sparse fields (ie. token is blank but app_token and bot_token are not, and the reverse. also the invalid combinations).