Refactor internal downloader code

[gboeing]

This PR:

• bumps the required requests version to 2.27 because we need its JSONDecodeError
• refactors the private _downloader module and extracts its code to work with the Overpass and Nominatim APIs into new _overpass and _nominatim modules
• refactors the elevation module
• handle cache_only_mode in the features module
• makes the features module stream downloads with a generator, like the graph module did in Stream downloads to save memory #1021 Better memory efficiency when downloading/constructing graph #1029 (see also Save memory by streaming more and duplicating less #1016)

[codecov]

Codecov Report

Attention: Patch coverage is 97.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 97.71%. Comparing base (55cc964) to head (e60b69d).
Report is 819 commits behind head on main.

Files with missing lines	Patch %	Lines
osmnx/features.py	61.53%	5 Missing ⚠️
osmnx/elevation.py	96.55%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1043      +/-   ##
==========================================
- Coverage   97.72%   97.71%   -0.01%     
==========================================
  Files          24       26       +2     
  Lines        2461     2456       -5     
==========================================
- Hits         2405     2400       -5     
  Misses         56       56              

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