Add new Adapter and Action interfaces, add new Next.js handler implementation, remove middlewares

[witoszekdev]

This PR adds new interfaces for writing handlers:

• Adapter
• Action
• Validator

It replaces Next.js handlers with these new abstractions.
It removes deprecated process-protected-handler, in favor of validator class.

It also removes middlewares and retes dependency, since they are no longer used by library code.

[changeset-bot]

🦋 Changeset detected

Latest commit: a3c50b2

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@saleor/app-sdk	Major

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[codecov]

Codecov Report

Attention: Patch coverage is 93.40314% with 63 lines in your changes missing coverage. Please review.

Project coverage is 79.81%. Comparing base (d339a69) to head (a3c50b2).

Files with missing lines	Patch %	Lines
src/handlers/shared/saleor-webhook-validator.ts	81.16%	29 Missing ⚠️
src/handlers/actions/register-action-handler.ts	97.16%	9 Missing ⚠️
.../handlers/shared/generic-adapter-use-case-types.ts	0.00%	8 Missing and 1 partial ⚠️
src/handlers/shared/generic-saleor-webhook.ts	94.82%	6 Missing ⚠️
src/test-utils/mock-adapter.ts	85.18%	4 Missing ⚠️
src/handlers/actions/index.ts	0.00%	1 Missing and 1 partial ⚠️
src/handlers/platforms/next/index.ts	0.00%	2 Missing ⚠️
src/handlers/platforms/next/platform-adapter.ts	97.36%	1 Missing ⚠️
...forms/next/saleor-webhooks/saleor-async-webhook.ts	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #401      +/-   ##
==========================================
+ Coverage   73.07%   79.81%   +6.74%     
==========================================
  Files          82       79       -3     
  Lines        3034     3156     +122     
  Branches      456      513      +57     
==========================================
+ Hits         2217     2519     +302     
+ Misses        812      630     -182     
- Partials        5        7       +2     

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