Add ALTER EXTENSION ADD/DROP ACCESS METHOD, and use it in pg_upgrade.

Without this, an extension containing an access method is not properly
dumped/restored during pg_upgrade --- the AM ends up not being a member
of the extension after upgrading.

Another oversight in commit 473b93287, reported by Andrew Dunstan.

Report: <f7ac29f3-515c-2a44-21c5-ec925053265f@dunslane.net>
