ft: Add slack alert profile

[someone-stole-my-name]

Description

Add support for Slack alert profiles

Motivation and Context

Currently, there's only support for raw webhook alerts in the provider but Prisma supports Slack webhooks too.

How Has This Been Tested?

On our own tenant with TF_ACC=1 go test -v ./internal/provider/... -run TestAccAlertProfileWebhook.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist

• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes if appropriate.
• [x] ~All new and existing tests passed.~ New tests pass, existing ones are broken 😢