feat: Add async functionality to providers

[leohoare]

This PR

Adds the ability for open feature providers to use async methods
It extends the single client and attempts to refactor some code

Related Issues
#284
#383
#385

Follow-up Tasks & TODOS

• Add tests for async provider and implementation
• Add documentation and how to implement an async hook

[codecov]

Codecov Report

Attention: Patch coverage is 91.95402% with 14 lines in your changes missing coverage. Please review.

Project coverage is 96.68%. Comparing base (cbace6a) to head (edc6908).

Files with missing lines	Patch %	Lines
openfeature/client.py	81.42%	13 Missing ⚠️
openfeature/provider/in_memory_provider.py	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #413      +/-   ##
==========================================
- Coverage   97.54%   96.68%   -0.87%     
==========================================
  Files          31       31              
  Lines        1387     1507     +120     
==========================================
+ Hits         1353     1457     +104     
- Misses         34       50      +16     

Flag	Coverage Δ
unittests	96.68% <91.95%> (-0.87%)	⬇️

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[beeme1mr]

Hey @leohoare, this looks good so far. Could you please add tests covering async providers with sync client calls and vise versa? Thanks for your hard work on this. 🍻