Could you point me to any documentation or examples? While I can find many plugins in the repo and around the Internet, I could find none which add authdb drivers.

https://dovecot.org/patches/2.2/passdb-openam.c although it's not using autotools to do it in a nice way. For the autotools stuff you can use some other plugins as example. Pigeonhole for example, although it's much more complicated than most. https://dovecot.org/patches/2.2/mail-filter.tar.gz probably has also (I didn't look into it).