add `buffer_geometry` helper function to `utils_geo` module

[gboeing]

This new function offers users a small quality-of-life improvement for use cases like:

• modeling a buffered graph around some study area (e.g., #1072, https://github.com/gboeing/osmnx/pull/1044#issuecomment-2020261305)
• retaining all edges that intersect a study area even if their endpoint nodes are all outside the study area (e.g., #1207)

For example, to download a graph within a 500 meter buffer of a city's boundaries:

import osmnx as ox
geom = ox.geocoder.geocode_to_gdf("Piedmont, CA, USA").iloc[0]["geometry"]
G = ox.graph.graph_from_polygon(ox.utils_geo.buffer_geometry(geom, 500), network_type="drive")

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.31%. Comparing base (94b7e73) to head (95846f2). Report is 4 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1214 +/- ## ======================================= Coverage 98.31% 98.31% ======================================= Files 24 24 Lines 2369 2374 +5 ======================================= + Hits 2329 2334 +5 Misses 40 40 ```

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