Extend maestro coverage command for graph output

[JenySadadia]

Enable bar graph output for maestro coverage data.

[JenySadadia]

The graph would look like below:
Command:
$ kci-dev --settings ./kci-dev.toml maestro coverage --start-date 2025-09-15 --graph-output

Note: Truncated commit IDs to have the first 6 chars.

[aliceinwire]

nice the graphs looks nice and is a good addiction to kci-dev, I completely agree on adding it as a optional feature as you did with the optional cmd argument.
This could be added/moved to a plugin module in the future (for not adding too many "optional" dependency to the main code), for now I think is good as is and I'm OK with merging.