Add go.mod for CLI package

[kian99]

Description

Adds a go.mod (and go.sum) file to the cmd/bakery-keygen folder to make it easier for users to generate a key-pair. Now one only needs to run go run github.com/go-macaroon-bakery/macaroon-bakery/cmd/bakery-keygen/v3

Testing

Tested to work via my fork, I changed the go.mod organisation to my user kian99 instead of go-macaroon-bakery and the above go run command worked. I am not using the below key, only showing it to prove the command worked.

$ go run github.com/kian99/macaroon-bakery/cmd/bakery-keygen/v3
{
	"public": "XpGi+QruMiunKzPBt2ewsO9RIXmLzOn8FbcrccKNE0E=",
	"private": "RY/RBkTykmJrSsjGTvzpc5k9lw8tcp63h1eOJ0levzo="
}