Resolve "Convert all output to using that client for `cmd/config.go`"

General MR

Summary

Changes output handling in bbctl config command to use the output client and adds formatting support for YAML, JSON, and TEXT config outputs.

Also adds unit tests for new config command format functionality.

Relevant logs/screenshots

Unit test coverage

Linked Issue

#140 (closed)

Upgrade Notices

Updates return type for MarshallHumanReadable interface definition from (string, error) to ([]byte, error) to match the return types for yaml and json.

Closes #140 (closed)