Widevine_test using my own KID:KEY values?

(Nice golang tool)
I took a code and compiled `examples/license.exe (license_example.go)` and run it. It worked fine, connected Google Widevine(test) license generator and printout KID and KEY values. This script puts a contentID value and autogenerated kid:key is returned from the widevine server.

**If I want to use a my own predefined KID:KEY values is it possible?** Usecase is encrypt a dash file with mp4box and put Playready(test) and Widevine(test) pssh drm fields.

All tracks would be encrypted using this single key. I know Google prefers using a separate key for audio and video tracks but this is just a test case CENC multidrm testing.

```
KID=0x43215678123412341234123412341237
KEY=0x12341234123412341234123412341237
Playready Test LAURL: https://test.playready.microsoft.com/service/rightsmanager.asmx?cfg=(kid:header,sl:2000,persist:false,firstexp:60,contentkey:EjQSNBI0EjQSNBI0EjQSNw==)
```

What would be Widevine LAURL url be specified in client player? Should I use `proxy_example.go` http app or public free test laurl server available somewhere? Microsoft provides a nice test plaready server.

PS:
I did few code changes: removed a deprecaetd policy field from request json, put SD_UHD2 track identifier as a max resolution.

widevine/license.go, row 195/setPolicy() `// "policy": policy.Policy`
widevine/license.go, row 207/buildLicenseMessage() `"allowed_track_types": "SD_UHD2"`
widevine/examples/license/license_example.go, row 39 `Tracks: []string{"SD", "HD", "AUDIO", "UHD1", "UHD2"}`