Add progress infos to TaskInfo

When querying for task's infos through the REST api, we get back a `percent` and `msg` fields when progress has been set.

The `TaskInfo` struct didn't reflect this, so I added them. The only issue with this is when there's no progress set, `percent` will be `0` and `msg` an empty string by default instead of not being included in the response like in the REST api.

Edit: fixed the issue about optional `percent` and `msg` fields with https://github.com/phacops/iron_go/commit/f437c5aee2125dc65ed5d13bdf814e2b1381bf50.