Store action code as attachments

See [dev thread][1] for background. All action code should be saved as attachments. This work would be done in following steps

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

[1]: https://lists.apache.org/thread.html/290fbc7cfa54071754afdb62b383018a52586a0953bddebcde500072@%3Cdev.openwhisk.apache.org%3E