Task logger

[henrysun007]

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context.

Fixes # (issue)

https://github.com/apache/incubator-teaclave/issues/669

The task logger part. Platform logging will be in another PR in a while.

Type of change (select or add applied and delete the others)

• [ ] Bug fix (non-breaking change which fixes an issue)
• [X] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] API change with a documentation update
• [ ] Additional test coverage
• [ ] Code cleanup or just sync with upstream third-party crates

How has this been tested?

Checklist

• [X] Fork the repo and create your branch from master.
• [X] If you've added code that should be tested, add tests.
• [X] If you've changed APIs, update the documentation.
• [X] Ensure the tests pass (see CI results).
• [X] Make sure your code lints/format.

[mssun]

The original design is to separate executors with other components to make it more flexible. In this implementation, if I understand correctly, executors are talking to the storage directly. Why not return the logs with the function return value instead?

[henrysun007]

Keeping the logs persistent is good for audit. The log is more a supplement to TaskState that is persistent in the storage service than just a diagnosis tool for developers here. Besides, the log content may be large so it is not very suitable as part of the result. However, as you have already been used to the result way in the cleanroom branch, we can use it in the master. Large log scenarios can be handled in the future.