Avoid optional Tracker attributes and enable mypy

[carmocca]

What does this PR do?

Refactor the progress structure to avoid the Optional[int] attributes. They made typing these files pretty tricky.

We had it previously to avoid users accidentally setting unused attributes. We now use separate classes so the attributes don't even exist.

Part of #5023

Does your PR introduce any breaking changes? If yes, please list them.

None user-facing.

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