chore: update PR template and auto-assign sdk-team reviewers

[joaodordio]

Summary

• Adopt the shared SDK PR template (Summary, Jira, Description, How to test, Loom, GH Issues, Docs PR).
• Add .github/workflows/assign-reviewers.yml to request review from @Iterable/sdk-team on new (non-draft) PRs, placing the team in the Reviewers section.
• Existing CODEOWNERS (@mprew97) left unchanged.

Test plan

• Open a test PR against this branch and confirm the new template renders.
• Confirm sdk-team is auto-requested as a reviewer once merged to the default branch.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.43%. Comparing base (6bbb6ce) to head (716dbc5).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #579   +/-   ##
=======================================
  Coverage   74.43%   74.43%           
=======================================
  Files          57       57           
  Lines        2742     2742           
  Branches      804      821   +17     
=======================================
  Hits         2041     2041           
  Misses        700      700           
  Partials        1        1           

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