Name of binary is `cli` but should be `openfeature`

[sahidvelji]

When installing via go install, the name of the binary is cli (as in, a user would write the command cli init instead of openfeature init), which I'm sure is not the intention. To fix this, the directory structure needs to be changed so that main.go is in ./cmd/openfeature/, and all files in cmd should be moved to be under ./cmd/openfeature/.

See for example the gh CLI: https://github.com/cli/cli