Implement a custom auth provider

As per the doc `authentication.txt`:
```The standard authenticator provided as a reference implementation uses
HMAC-SHA-256 to compute the HMAC of the request, with the hex-encoded
authentication key specified in the configuration file. The key may be
specified in one of three ways:

* hex-encoded string (e.g. "000102030405060708")
* an environment variable prefixed with "env:"
(e.g. "env:AUTH_KEY") that contains a hex-encoded string.
* a path to a file containing the hex-encoded key, prefixed with
"file:" (e.g. "file:/path/to/auth.key")
```

If one had to implement a custom auth provider, one updates the `authTypes` map for calling the right auth provider in `cfssl_provider.go`.

What I do not seem to understand is how do I update this private, non-global map to include a custom auth provider?

The use case is to provide a dynamic auth provider for clients, which ensure we do not change CFSSL configs repeatedly (but can easily change some internal systems to do so).