Re: [push-api] A lower level version of #180 (#182)

LGTM

I prefer using a BufferSource because (a) the UA generally won't have to do anything with the key, and (b) there is no ubiquitous support for uncompressed EC points across Web Crypto implementations.

I'm guessing that the common case will become something like:

```javascript
const APPLICATION_SERVER_KEY = new Uint8Array([
    0x04, 0x00, 0x00, 0x00, ...
]);
```

Which is much more developer friendly than loading a CryptoKey object (which, in Chrome, requires converting to a JWK object prior to importing :-/).

Thank you! :)

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