Add Declaration URI rewrite for referenced policies

[npearce]

AS3 support pulling policing in via URI, e.g. a WAF policy or an iRule:

"Maintenance_iRule": {
    "class": "iRule",
    "iRule": {
        "url": "https://webserver.com/irule/maintenance-page.irule"
    }
}

Need to support URL rewrite option to fetch objects from {{source_repo}}:
https://{{source_repo}}/maintenance-page.irule

[npearce]

Look for "url" key with value containing webhook:// prefix, e.g.:

"url": "webhook://[owner/repo]/[as3 referenced asset]"

Webhook server will replace this with 'Github raw' link to referenced asset (irule, waf_policy, etc).