Add support for managing custom SSL certificates

[khawarizmus]

Problem Statement

Currently, there is no way to manage custom SSL certificates (like Cloudflare SSL certificates) through the Netlify Terraform provider. The manual process of adding certificates through the Netlify domain management UI is:

• Time-consuming
• Error-prone
• Difficult to automate
• Inconsistent with Infrastructure as Code principles

Proposed Solution

Add a new resource to manage SSL certificates, potentially with a structure like:

resource "netlify_ssl_certificate" "cloudflare" {
  site_id     = netlify_site.example.id
  certificate = file("path/to/certificate.pem")
  private_key = file("path/to/private.key")
}