feat: add create method to handle token headers

[silentworks]

What kind of change does this PR introduce?

Add a new create method to handle async token retrieval

What is the current behavior?

The library isn't fully async and complains when trying to call get_session inside of _get_token_header from a @property.

What is the new behavior?

The library should now be fully async as the get_session can be called async since we've moved the _get_token_header to the new create method instead of calling it from a @property.

Additional context

Add any other context or screenshots.

[codecov]

Codecov Report

Attention: 28 lines in your changes are missing coverage. Please review.

Comparison is base (b9240d8) 46.15% compared to head (4f47306) 45.22%.

Files	Patch %	Lines
supabase/_async/client.py	0.00%	18 Missing ⚠️
supabase/_sync/client.py	50.00%	8 Missing ⚠️
supabase/client.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #630      +/-   ##
==========================================
- Coverage   46.15%   45.22%   -0.94%     
==========================================
  Files          13       13              
  Lines         299      314      +15     
==========================================
+ Hits          138      142       +4     
- Misses        161      172      +11     

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[J0]

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