Store arrow schema when reading catalogs

[camposandro]

When instantiating a catalog, extract the arrow schema from the metadata files and store it.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.84%. Comparing base (047600e) to head (6977e02).
Report is 37 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #310      +/-   ##
==========================================
+ Coverage   93.78%   93.84%   +0.06%     
==========================================
  Files          58       58              
  Lines        2028     2048      +20     
==========================================
+ Hits         1902     1922      +20     
  Misses        126      126              

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[github-actions]

Before [047600e] <v0.3.7>	After [6ce811e]	Ratio	Benchmark (Parameter)
41.7±0.3ms	42.6±0.8ms	1.02	benchmarks.Suite.time_pixel_tree_creation
13.5±0.2ms	13.6±0.2ms	1.01	benchmarks.Suite.time_inner_pixel_alignment
384±2ms	383±4ms	1	benchmarks.Suite.time_outer_pixel_alignment
119±0.7ms	119±0.5ms	1	benchmarks.time_test_alignment_even_sky
20.9±0.5ms	20.8±0.4ms	0.99	benchmarks.MetadataSuite.time_load_partition_info_order6
78.2±2ms	77.7±0.4ms	0.99	benchmarks.MetadataSuite.time_load_partition_join_info
980±3μs	974±20μs	0.99	benchmarks.time_test_cone_filter_multiple_order
79.2±1ms	77.0±0.3ms	0.97	benchmarks.MetadataSuite.time_load_partition_info_order7
91.3±2ms	86.9±2ms	0.95	benchmarks.Suite.time_paths_creation

Click here to view all benchmarks.

[delucchi-cmu]

Should we add this for index catalogs as well? Does it just not matter? They don't inherit from HealpixDataset, just that superclass Dataset.