Store action code as attachments

See dev thread for background. All action code should be saved as attachments. This work would be done in following steps

Support storing action code as attachment as per manifest (Treat action code as attachments #3945)
Remove attachment information from manifest and always store code as attachment
Support attachments for blackbox action (Treat blackbox action code as attachment #3979)
Simplify existing code to remove parallel hierarchy of MetaData classes for various Exec types

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