Record fixed update start time in client workflow

[sechkova]

Description of issue or feature request:

Record fixed update start time as per the specification:
https://github.com/theupdateframework/specification/blob/master/tuf-spec.md#record-fixed-update-start-time--fix-time

Sync with: #1308

[jku]

This will be fixed by #1355

[jku]

Implemented 🥇