Set serverCertificate via Reference UI

[dsilhavy]

Description

The EME provides a method to set a server certificate: https://www.w3.org/TR/encrypted-media/#dom-mediakeys-setservercertificate

dash.js exposes this API via the ProtectionController and also allows application to provide the certificate within the protectionData:

• https://cdn.dashjs.org/latest/jsdoc/module-ProtectionController.html#setServerCertificate

        var protData = {
            "com.widevine.alpha": {
                "serverURL": "https://someurl.com",
                "httpRequestHeaders": {
                    "X-AxDRM-Message": "somevalue"
                },
               serverCertificate: "somedata",
                priority: 0
            }
        };

It should be possible to provide a server certificate via the dash.js Reference UI as part of the DRM options.