Credential Provisioning for SFT authentication

[stefanwire]

https://wearezeta.atlassian.net/browse/WPB-227

Checklist

• [ ] create additional configuration for brig
• [x] create username and credential fields in sft_servers_all
• [ ] rate-limit new credentials by user
• [ ] include is_federating
• [ ] Add a new entry in an appropriate subdirectory of changelog.d
• [ ] Read and follow the PR guidelines