feat: add public download + no_auth method features

[cboudereau]

fixes #29

• [x] add public download example
• [x] add no_auth method to download without any configured gcs auth
• [x] add no_auth method to gcs-rsync cli

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 98.49624% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 89.23%. Comparing base (2d34bfe) to head (a52e56c). Report is 1 commits behind head on main.

Files	Patch %	Lines
src/gcp/storage/client.rs	95.23%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #32 +/- ## ========================================== + Coverage 88.91% 89.23% +0.31% ========================================== Files 14 16 +2 Lines 2328 2396 +68 ========================================== + Hits 2070 2138 +68 Misses 258 258 ```

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[trevorbernard]

Awesome!