Refactor schema backends

[cosmicBboy]

This PR refactors the api package schemas to use the corresponding backends:

• Add BaseSchema.register_backend and get_backend methods.
• Refactor how backend core checks work by outputting a CoreCheckResults named tuple instead of raising a SchemaError
• Clean up the core check invocations in the backend schema validate methods
• Clean up the SchemaErrors.collect_error interface
• Introduce BackendNotFoundError exception type

[codecov]

Codecov Report

Patch coverage: 96.96% and project coverage change: -0.09 ⚠️

Comparison is base (76bfa4f) 97.45% compared to head (03cb986) 97.37%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1144      +/-   ##
==========================================
- Coverage   97.45%   97.37%   -0.09%     
==========================================
  Files          67       65       -2     
  Lines        4840     4912      +72     
==========================================
+ Hits         4717     4783      +66     
- Misses        123      129       +6     

Impacted Files	Coverage Δ
pandera/backends/pandas/components.py	96.66% <68.75%> (-2.66%)	⬇️
pandera/backends/pandas/array.py	98.11% <95.83%> (-0.97%)	⬇️
pandera/backends/pandas/container.py	98.23% <98.43%> (+0.49%)	⬆️
pandera/__init__.py	89.47% <100.00%> (+1.23%)	⬆️
pandera/api/base/checks.py	100.00% <100.00%> (ø)
pandera/api/base/schema.py	100.00% <100.00%> (ø)
pandera/api/pandas/array.py	100.00% <100.00%> (ø)
pandera/api/pandas/components.py	99.11% <100.00%> (-0.03%)	⬇️
pandera/api/pandas/container.py	99.25% <100.00%> (-0.01%)	⬇️
pandera/backends/base/__init__.py	100.00% <100.00%> (ø)
... and 6 more

... and 3 files with indirect coverage changes

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