Fix endpoint url in STAC item's asset published by xcube server

[konstntokas]

The endpoint_url was wrongly assigned to "http://localhost:8080/s3", which prevents data access. This Pr fixes this issue by parsing in the base url.

Checklist:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/source/*
• Changes documented in CHANGES.md
• GitHub CI passes
• AppVeyor CI passes
• Test coverage remains or increases (target 100%)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.51%. Comparing base (7d05c11) to head (cdf3632).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1178   +/-   ##
=======================================
  Coverage   89.51%   89.51%           
=======================================
  Files         280      280           
  Lines       21659    21659           
=======================================
  Hits        19387    19387           
  Misses       2272     2272           

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