Seperate OSSFile from core to file.py

[karajan1001]

fix #91

1. As the project gets more and more complex, can not keep All things in a single file.
2. Update some typing hints.

[codecov-commenter]

Codecov Report

Patch coverage: 92.98% and project coverage change: +0.05 :tada:

Comparison is base (f27d57a) 94.06% compared to head (5196c28) 94.11%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #102 +/- ## ========================================== + Coverage 94.06% 94.11% +0.05% ========================================== Files 8 9 +1 Lines 1162 1173 +11 Branches 182 182 ========================================== + Hits 1093 1104 +11 Misses 42 42 Partials 27 27 ``` | [Impacted Files](https://codecov.io/gh/fsspec/ossfs/pull/102?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [src/ossfs/file.py](https://codecov.io/gh/fsspec/ossfs/pull/102?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL29zc2ZzL2ZpbGUucHk=) | `85.18% <85.18%> (ø)` | | | [src/ossfs/\_\_init\_\_.py](https://codecov.io/gh/fsspec/ossfs/pull/102?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL29zc2ZzL19faW5pdF9fLnB5) | `100.00% <100.00%> (ø)` | | | [src/ossfs/core.py](https://codecov.io/gh/fsspec/ossfs/pull/102?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL29zc2ZzL2NvcmUucHk=) | `84.46% <100.00%> (+0.41%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None)

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