feature: Adding Signature verification for webhooks

[kvilesjo]

Current webhooks supports bearer token as authentication method and it is a required header in order to start a workflow or script. Some could services that supports webhook integration supports signature verification instead of or in addition to the bearer token.
The input variables from a web hook trigger do not include the header or the raw body needed to validate the signature.
The windmill custom http route may be used for this purpose with a preprocessor step to validate the signature before forwarding the webhook data to the script or workflow. https://www.windmill.dev/docs/core_concepts/http_routing

Please add a script for signature validation in the preprocessor step. It can generate public/private key pair or take one as input. The header field name for the signature may also be an input to the method.