feat: add setRequestToken and fetchRequestToken methods

[susnux]

Basically move them from server to this library, implementation only slightly changed for the non-dom case to keep service worker support. Otherwise same implementation and even tests are copied.

This allows us to have 1 place with the set token logic to reduce problems where the tokens get out of sync.

[codecov]

Codecov Report

❌ Patch coverage is 91.42857% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.00%. Comparing base (4d62736) to head (669418f).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/requestToken.ts	90.62%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #900      +/-   ##
==========================================
+ Coverage   60.71%   70.00%   +9.28%     
==========================================
  Files           4        4              
  Lines          56       80      +24     
  Branches       17       23       +6     
==========================================
+ Hits           34       56      +22     
+ Misses         20       19       -1     
- Partials        2        5       +3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[susnux]

(rebased)

[szaimen]

🐘

[pringelmann]

looks good, just minor comment on subscription cleanup

[cristianscheid]

🐘