feat: handling for partial downlads

[coanghel]

• [x] Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• [x] Ensure the tests and linter pass Used nox -s scripts-3.9 , also manually ran test_http.py
• [x] Code coverage does not decrease (if any source code was changed) No change before/after with scripts-3.9
• [x] Appropriate docs were updated (if necessary)

Added an optional start parameter to the constructor of MediaIoBaseDownload() to allow for resuming a download from a certain byte. If not provided, MediaIoBaseDownload() will continue to function as it does currently and start the download from byte 0.

Fixes #2309 🦕

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