Key system priority

[bbert]

The goal of this PR is to enable the key systems selection process to request the key systems access in a given priority order.

By default, in dash.js the key systems order is pre-defined in ProtectionKeyController:

dash.js/src/streaming/protection/controllers/ProtectionKeyController.js

Line 72 in f4418b5

function initialize() {

))

As a use case, on some devices you may have 2 different available CDMs, and for a specific service you would require dash.js to select the second one in the pre-defined order.
Thus the user can set the priority in the protectionData in order to force dash.js selecting the key systems in the user-defined order.

For example, if you provide the following protectionData:

{
    "com.widevine.alpha": {
        "serverURL": "https://drm-widevine-licensing.axtest.net/AcquireLicense",
        "priority": 0
    },
    "com.microsoft.playready": {
        "serverURL": "https://drm-playready-licensing.axtest.net/AcquireLicense",
    }
}

Then dash.js will select first and request access to the Widevine key system.

The same goald could have been achieved based on PR #2953, but this PR propose a more easiest way to achieve this.