added external custom domain support to Api construct

[Manitej66]

Added support for adding custom domains to the Api construct that are outside of Route 53.

Sample code example:

const api = new sst.Api(this, "Api", {
  customDomain: {
    domainName: "api.sst.sh",
    isExternalDomain: true,
    certificate: acm.Certificate.fromCertificateArn(
      this,
      "Certificate",
      "arn:aws:acm:us-east-1:000639264671:certificate/xxxx"
    ),
  },
  routes: {
    "GET /": "src/lambda.main",
  },
});

[netlify]

✔️ Deploy Preview for sst-console canceled.

🔨 Explore the source changes: 8f1c92f

🔍 Inspect the deploy log: https://app.netlify.com/sites/sst-console/deploys/61e345c0c89e8e000816aae8

[JonHolman]

Thank you @Manitej66 and @fwang