Add support for MariaDB Foundry to buildbot.

This needs three parts: 11.4, the foundry, and buildbot. First two are done, but not considered final. 11.4 part is in https://github.com/MariaDB/server/tree/bb-11.4-pkgtest-MDEV-40608, the foundry is in https://github.com/vuvova/foundry/. Buildbot will be dev, until all parts are proven to work well together.

How it works:

• buildbot clones foundry (it's very small) and builds all plugins it on all server release builders and all server versions starting from 11.4
  • this job is triggered only manually, not on push
  • it should save the packages
• buildbot clones foundry and builds only one plugin
  • this is done on a push to the foundry, when an individual plugin's file is modified
  • it works as a branch protection, allowing a PR to be merged
• to build plugins the builder only needs MariaDB-devel or libmariadb-dev not the server sources. This devel package must be taken from the last main (or -release?) branch build for the same version.
• some plugins include tests. After they're built an install/test step would need MariaDB-test too. But not for build.