Support both 'flags' and 'ConfigFlags' in pre/postExec

Tadej Novakrequested to merge
tadej/athena:flags-prepostexec into master

Support both flags and ConfigFlags in pre/postExec. The motivation for this is that we're trying to achieve flags as de-facto variable for non-global configuration flags instance. Keep ConfigFlags for backwards compatibility for now.

Also add tests for that to ensure it actually works as expected. More tests for TransformUtils will be added separately.

/cc @fwinkl @jchapman

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