Add `leo report` hooks on the user script

[0x2b3bfa0]

Not tested, may need heavy reformatting. To be cascaded into #733.

Statuses

• queued — waiting for a new spot instance
• running — the user-specified script started to run
• timeout — terminated after exceeding the execution time limit
• succeeded or failed — finished voluntarily with [non-]zero exit status

Note when a new task is created, its status should be hardcoded to queued until machines submit their first status report.

[0x2b3bfa0]

Closing along with #733, superseded by #735 🙃