amp-amazon-onetag: Amazon affiliate links

## What's the issue?

Its a feature request. I'm part of Amazon's Affiliate Marketing team. Amazon affiliate marketing is a channel that helps the content creators monetize their content by paying a fraction of profit for purchases from traffic coming from their websites.

We've a script called OneTag, which associate can add to their pages. Some benefits of OneTag script are:
1. It tags the amazon links on the page automatically which ensures that any click coming to amazon is attributed to the associate.
2. It provides feature to redirect the user clicking the link to amazon retail store based on the geo location of the user.
3. Provides data to associates to give them insight into their content performance.

We've observed a high adoption of Amazon associates on AMP pages and as a result we'd like to add the support for OneTag script on Amp pages.

**Sample code:**
```javascript

export class AmpAmazonOnetag extends AMP.BaseElement {
/** @param {!AmpElement} element */
constructor(element) {
super(element);
this.document = element.ownerDocument;
}

/** @override */
buildCallback() {
const onetagEndpoints = new Map();
onetagEndpoints.set("US", "z-na.associates-amazon.com");
onetagEndpoints.set("GB", "z-eu.associates-amazon.com");

var marketplace = this.element.getAttribute('marketplace');
var instanceID = this.element.getAttribute('instanceID');
if (onetagEndpoints.has(marketplace)) {
var scriptURL = "https://" + onetagEndpoints.get(marketplace) + "/widgets/onejs?MarketPlace=" + marketplace + "&adInstanceId=" + instanceID;
loadScript(this, scriptURL);
}
}

/** @override */
isLayoutSupported(layout) {
return layout == Layout.NODISPLAY;
}
}

AMP.extension('amp-amazon-onetag', '0.1', AMP => {
AMP.registerElement('amp-amazon-onetag', AmpAmazonOnetag);
});
```
**Example:**
```html

amp-amazon-onetag example

Sample Amazon link

```