Reject > 1 bucket / key separator

[MrCreosote]

Note: this is backwards incompatible but unlikely to cause issues

• Minio collapses multiple slashes into a single slash in bucket/key paths
• CEPH does not
• Posix files systems more or less collapse them

Putting multiple slashes in an object is weird anyway, so just throw an error on any paths with multiple slashes. The current code already does this for internal multi-slashes in keys, just do it for the start of keys as well rather than silently collapsing them.

Also add the checksum algorithm to the file uploader test fixture

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 53.33%. Comparing base (9fac477) to head (9441195).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #576      +/-   ##
==========================================
+ Coverage   53.29%   53.33%   +0.03%     
==========================================
  Files          69       69              
  Lines        5965     5972       +7     
==========================================
+ Hits         3179     3185       +6     
- Misses       2786     2787       +1     

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