Add a way to configure the default multibase used

### Checklist

- [X] My issue is specific & actionable.
- [X] I am not suggesting a protocol enhancement.
- [X] I have searched on the [issue tracker](https://github.com/ipfs/go-ipfs/issues?q=is%3Aissue) for my issue.

### Description

After #8886.

We could configure what the default multibase is, for example myself I work mainly in the terminal and prefer `base64url` because it's shorter and pollute less of my screen.

I think the best way to do that would be an environment variable that the client read.
(there is no reason to force that config global to the node, however we don't have any client only config file and I don't think we need one).

Simply a simple:
```
export IPFS_DEFAULT_MULTIBASE=base256emoji
```
would be enough.

This flag would be used as *default*. If a command has a good reason to override it, such as `ipfs cid format -b base32` for example, it would be ignored.