Handle credentials rotation

[emmaai]

Changes:

• removed the static credentials, let boto3.client handle the credentials rotation
• changed how s3_client (ref:Gather s3 operations into class odc-tools#597) is managed on dask cluster, one for scheduler and one per work (among all threads rather than each per thread)

[SpacemanPaul]

Looks good, but obviously can't be merged until the changes in odc-cloud are released.

[codecov]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (ec3cdd3) 76.38% compared to head (6e23284) 76.59%.
Report is 2 commits behind head on develop.

Files	Patch %	Lines
odc/stats/io.py	54.54%	5 Missing ⚠️
odc/stats/proc.py	55.55%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #114      +/-   ##
===========================================
+ Coverage    76.38%   76.59%   +0.21%     
===========================================
  Files           34       34              
  Lines         3210     3205       -5     
===========================================
+ Hits          2452     2455       +3     
+ Misses         758      750       -8     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.