Support for ETag and If-Not-Modified in GBFS updater

[testower]

Summary

This PR adds support for keeping ETag header values from GBFS datasource responses, and passing them to subsequent requests, in order for the server to respond with 304 Not Modified when the content hasn't changed.

If the responses don't have an ETag header, there is no change in behavior.

Issue

Closes #6927

Unit tests

All existing tests pass. No additional tests added as changes are mostly based around the http client. The ETag/if-not-modified behavior has been manually tested against Entur's gbfs aggregator.

[codecov]

Codecov Report

❌ Patch coverage is 44.11765% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.23%. Comparing base (4182f3e) to head (544e487).

Files with missing lines	Patch %	Lines
...le_rental/datasources/gbfs/GbfsFeedLoaderImpl.java	44.11%	12 Missing and 7 partials ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             dev-2.x    #6963      +/-   ##
=============================================
- Coverage      72.24%   72.23%   -0.01%     
- Complexity     19968    19972       +4     
=============================================
  Files           2163     2163              
  Lines          80365    80394      +29     
  Branches        8114     8120       +6     
=============================================
+ Hits           58061    58076      +15     
- Misses         19443    19452       +9     
- Partials        2861     2866       +5     

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