Dvc: config: add --list to list all variables and their values

[efiop]

Something that would be handy for us during debugging and for users. Same as git config --list. Useful to avoid confusion such as in https://github.com/iterative/dvc/issues/4970

[johnnychen94]

a quick question: should it print out the credential info, e.g., aws key/secret?

[efiop]

@johnnychen94 dvc config is pretty primitive, if you use dvc config remote.myremote, it will spit out the password too. It is probably not worth overcomplicating such a simple command. IIRC git config will also print passwords no questions asked.