Adapt BIP21 payment requests for Liquid

[BIP0021](https://github.com/bitcoin/bips/blob/master/bip-0021.mediawiki) defines the rules for payment requests, we need something similar for Liquid (or other elements instances), thus we should define:
- the name of the network prefix, for Liquid specifically `liquid` is proposed as preferred name
- additional parameters
- assetid, to refer to the user desired asset
- amount, amounts should probably follow [BIP21 guidelines](https://github.com/bitcoin/bips/blob/master/bip-0021.mediawiki#transfer-amountsize) and be specified in the URI as decimal BTC, irrespectively of the issuer defined precision ([examples](https://assets.blockstream.info/)). Though the clients – both when generating and parsing the URI – should keep in consideration the `precision` specified by the asset issuers, as that is the amount that issuers ultimately inted to be displayed to users.

I wonder if this is compatible with elements-qt, especially the assetid part, as the amount would be treated the same as Bitcoin already does.

Anyone have different ideas/objections to this?