feat/prefix: add prefix support

[cboudereau]

• [x] add prefix support
• [x] backlash are converted to slash to avoid synchronisation mess on remote gcs which supports slash as folder separator :
  • use case: 2 way merge with windows and linux os.

gcs-rsync will not add the extra slash in the prefix anymore to fully support prefix and partial sync.

Previously, gcs-rsync assumed that prefix was always a folder (by adding an extra slash at the end of prefix search) while with this new feature, the folder is inferred (by detecting the current folder with last slash) to be stripped from the relative path.

• Synchronise /hello/world prefix to /dest prefix files looks like:
  • Before this feature
  • /hello/world2.txt > does not match /hello/world/ (with the extra slash defaulted by gcs-rsync)
  • /hello/world/test.txt > /dest/test.txt
  • /hello/text.txt > not synced since it does not match the source prefix.
  • After this feature
  • /hello/world2.txt > /dest/world2.txt (now synchronized)
  • /hello/world/test.txt > /dest/world/test.txt (world subfolder is added)
  • /hello/text.txt > not synced since it does not match the source prefix.

Backward compatibility: simply add a slash at the end of the prefix and the behavior will remains same as before:

• Now, synchronising with /hello/world/ prefix to /dest prefix files is like before:
  • /hello/world2.txt > does not match /hello/world/ (with the extra slash defaulted by gcs-rsync)
  • /hello/world/test.txt > /dest/test.txt
  • /hello/text.txt > not synced since it does not match the source prefix.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 97.72727% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 89.12%. Comparing base (ad96a86) to head (ee6f466).

Files	Patch %	Lines
tests/config/gcs.rs	81.25%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #34 +/- ## ========================================== - Coverage 89.23% 89.12% -0.11% ========================================== Files 16 16 Lines 2405 2473 +68 ========================================== + Hits 2146 2204 +58 - Misses 259 269 +10 ```

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