manager: configurable Meson directory

This commit makes it more convenient to change the configuration of the build directory of kresd when using Knot Resolver Manager. It adds a new ./poe configure command, which optionally takes the same arguments as the standard meson configure command.

The ./poe run command now requires running ./poe configure at least once to set up the build directory. If the directory has been configured before this commit (i.e. ./poe run has been executed at least once), no extra action is required, as the directory structure remains the same.

The commit also removes the manager configuration option from Meson as we were not using it and it was broken and potentially confusing to newcomers.