databases/adminer: Ship plugins within the package

Adminer's plugins are now part of the same file and they're autoloaded
if used.
Add build-time options for database drivers and remove mentions about
drivers that aren't in the tree anymore.
MySQL and SQLite on by default due to their rather light dependency
chains.

PR: 288750
(cherry picked from commit 7e1d58ce98678ae2beefeb0806b1d02e7d164156)