add webhook validation method

[criskell]

• [x] Add an method on Webhooks resource for checking if an given webhook signature is valid, returning true or false.
• [x] Add docs for this method on https://github.com/Open-Pix/openpix-developers.

Useful resources:

• https://developers.openpix.com.br/docs/sdk/php/sdk-php-resources#validar-um-webhook
• https://developers.openpix.com.br/docs/webhook/webhook-signature-validation

Examples:

• https://github.com/Open-Pix/php-sdk/pull/34
• https://github.com/Open-Pix/ruby-sdk/pull/19

[aripiprazole]

https://github.com/Open-Pix/openpix-developers/pull/588