WAF: Allow rules to specify body parser type

[nateweller]

Resolves https://github.com/Automattic/jetpack-scan-team/issues/1423

Proposed changes:

1. Adds a public $version property to Waf_Runtime. The intention is for rule files to use this property, i.e. if ( ( $waf->version ?? 0 ) >= 5 ) { .. }.
2. Adds a private string $body_processor property to the Waf_Runtime class, along with a public function set_body_processor( $processor ) to enable rule files the ability to manually specify the method by which to retrieve post values from the request.
3. Updates the meta() method in Waf_Runtime to provide $this->body_processor to the $this->request->get_post_vars() method, which has been updated to support that new argument.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

https://github.com/Automattic/jetpack-scan-team/issues/1423

Does this pull request change what data or activity we track or use?

No

Testing instructions:

• Test that requests are parsed and blocked correctly when containing a property with a string value of <script>.
• Manually insert $waf->set_body_processor( 'JSON' ); at the top of your automatic rules file and repeat the test with a JSON body in your request, with any content type.
• Repeat with $waf->set_body_processor( 'URLENCODED' ); and repeat the test with applicable request body and content types.

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WordPress.com Simple site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin, and enable the add/waf/body-processor branch.

• To test on Simple, run the following command on your sandbox:

  bin/jetpack-downloader test jetpack add/waf/body-processor
  

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions]

Thank you for your PR!

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[marcS0H]

LGTM. 👍