api/cli: introduce remote_config

[efiop]

We already have config as the most powerful option, but remote_config would be convenient in most cases. As a start, it is fine to only make it set the default remote's config and not care about per-output remotes (one could use config if he needs to tweak that).

e.g. this will merge {"passphrase": "mypass"} with the default remote's config

with dvc.api.open(url, path, remote_config={"passphrase": "mypass"}):
....

this will set default remote to myremote and then merge {"passphrase": "mypass"} with myremotes remote's config

with dvc.api.open(url, path, remote="myremote", remote_config={"passphrase": "mypass"}):
....

[dberenbaum]

Let's use the same format as dvc exp run --set-param key=val

[efiop]

@dberenbaum You probably mean --remote-config for CLI, right? In api we can just pass a dict.