Add SQL support

Introduces the "sql" backend, and refactors backends behind a nicer
interface (which looks a lot like the one in id/auth). Users can
define arbitrary SQL queries to fetch the key data, to adapt to their
database schema.

Include vendored dependencies for sqlite3/mysql/postgres.
3 jobs for sql in 6 minutes and 4 seconds
Status Job ID Name Coverage
  Build Pkgsrc
passed #7243
build_pkgsrc:stretch

00:00:21

 
  Build Pkg
passed #7244
amd64
build_pkg:stretch:amd64

00:01:13

passed #7245
arm64
build_pkg:stretch:arm64

00:05:43