nextjs: implement SSR functions to retrieve user and access token info

[frankie567]

Following discussions in #8, those new functions should allow to retrieve user state information on the server-side; which will prove especially useful for SSR and App Router.

Most of the underlying mechanism was already in place with the middleware setting request headers.

Things yet to solve:

• User cache information for Edge runtime. This is required so we can set the full user information. We need to be careful of the Next.js behavior of automatically caching fetch requests
• Provide a way to pre-populate the React FiefAuthProvider, so client components can start with a pre-existing state coming from the server

[codecov]

Codecov Report

Attention: Patch coverage is 53.65854% with 19 lines in your changes missing coverage. Please review.

Project coverage is 86.26%. Comparing base (94d515b) to head (2977b29).

Files	Patch %	Lines
src/nextjs/index.ts	13.63%	19 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #10      +/-   ##
==========================================
- Coverage   88.84%   86.26%   -2.58%     
==========================================
  Files          10       10              
  Lines         529      568      +39     
  Branches      128      151      +23     
==========================================
+ Hits          470      490      +20     
- Misses         59       78      +19     

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