[authentication] named scanner broke auth factory

[cedricbu]

The authentication factory expected a fixed string for the scanners, to get to the authentication type.

This was broken by the named scanner (e.g.: zap_anonymous).

This commit fixes that.

Also, while at it, I tidied a little:

• moved the factory into its own file
• improved the dev documentation for authentication

[jeremychoi]

LGTM