Gaia: update the output file name built by the method load_data to include microsecond resolution

[cosmoJFH]

In #3128 describes the requirement to include the microsecond resolution in the output file name built by the method load_data.

Units tests were updated.

cc @esdc-esac-esa-int

jira: GAIAPCR-1331

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 67.53%. Comparing base (58f8692) to head (ce16157).
Report is 42 commits behind head on main.

Files with missing lines	Patch %	Lines
astroquery/gaia/core.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3130      +/-   ##
==========================================
+ Coverage   67.49%   67.53%   +0.04%     
==========================================
  Files         233      233              
  Lines       18413    18487      +74     
==========================================
+ Hits        12428    12486      +58     
- Misses       5985     6001      +16     

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[bsipocz]

Thanks @cosmoJFH, this all looks good to me. I'll leave a changelog label as a reminder for the release time to consolidate the entries.