Add dry-credentials and bridgetown_credentials

[svoop]

Thanks for contributing to the Ruby Toolbox catalog! <3

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  that it is not packaged as a Ruby gem. (If it is packaged as a gem, please reference it
  by its gem name instead, e.g. rails.)
• Please make sure the projects are listed in case-insensitive alphabetical order
• Make sure the CI build passes, we validate your proposed changes in the test suite :)

[colszowka]

Thanks @svoop 👍