Add a --corp cli flag similarly to --cors #817

Description

Add a --corp flag that would result in the equivalent of

{
  "headers": [
    {
      "source": "**/*",
      "headers": [
        {
          "key": "Cross-Origin-Resource-Policy",
          "value": "cross-origin"
        }
      ]
    }
  ]
}

that now has to be set up in a serve.json .

Why

Browsers now require CORP and COEP and the --cors flag is not enough for quickly starting up a server and testing things.

Alternatives

Add the Cross-Origin-Resource-Policy: cross-origin header by default when --cors is specified.