Add resource for custom rules

[hi-artem]

Description

Add custom rules resource to provider. This requires an update to pcc client, tracked by this pr https://github.com/PaloAltoNetworks/prisma-cloud-compute-go/pull/27

Motivation and Context

This feature is requested in #29

How Has This Been Tested?

I used the following resource to test changes:

resource "prismacloudcompute_custom_rule" "test" {
  name        = "basic-rule"
  description = "this is basic rule"
  message     = "%proc.name doing stuff"
  type        = "processes"
  script      = "proc.name = \"cat\""
}

resource "prismacloudcompute_custom_rule" "test_heredoc" {
  name        = "less-basic-rule"
  description = "this is less basic rule"
  message     = "%proc.name wrote to path"
  type        = "filesystem"
  script      = <<EOT
                  // Example:
                  // user modifies a sensitive file under /etc or its subfolders
                  // proc.user != "root" and file.path startswith "/etc"

                  proc.user != "crond" and file.path startswith "/var/spool"
                EOT
}

Types of changes

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

Checklist

• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes if appropriate.
• [ ] All new and existing tests passed.