[FEATURE] Explicit schedule execution

[pilosus]

Is your feature request related to a problem? Please describe.
When a scheduling worker executes a scheduled command, it's done "silently". The bot just sends a message of the executed command, but never tells it's done so because of the schedule. It infringes the principle of least surprise.

Describe the solution you'd like
Send an extra message to the chat before executing a scheduled command, e.g.

message 1: Executing `<executable>` with `<crontab>` schedule
message 2: <results of execution>