[w3c/push-api] User Agents should report accepted content-encoding types. (#251)

Recently, a bug filed against a webpush subscription library highlighted a shortcoming.

https://github.com/web-push-libs/web-push-php/issues/48#issuecomment-295416292

Currently, there are two in production encryption encoding types, "aesgcm" and "aes128gcm". The "voice of authority" about what types of accepted content types is the UA. The sorts of allowed encryption is not communicated to the subscription update provider.

I would like to propose that the returned [PublishSubscription](https://developer.mozilla.org/en-US/docs/Web/API/PushSubscription) object  `options` object be modified to include a "encodingtypes" list of allowed ECE encoding types. (e.g. `["aesgcm", "aes128gcm"]`) This method would also allow future content types to be relayed. If no "encodingtypes" field is present, then the provider must assume "aesgcm" encoding, to allow for older UAs.

This field would also help indicate "updated" UAs which can take advantage of the newer draft specifications.

(Some additional, albeit incorrectly located, [discussion is here](https://mailarchive.ietf.org/arch/msg/webpush/uDR2PzpChfn24TwNWZBfR6fuKp0).) 

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