BIP352: Add extra encoding formats for scanning and spending

[junderw]

Based on a discussion I started on Delving Bitcoin

Also related: BTC Transcripts

BIP authors:

@josibake @RubenSomsen

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These two new encodings are a way for Silent Payment users to:

1. Delegate scanning and wallet UTXO management to self hosted services like BTCPayServer
2. Encode secret information including birthday and labels used, so that silent payment wallets can be exported and imported across wallets without information loss.

I was debating whether to make the non-address encodings into descriptors instead. But that would be an easy change to make.

Just creating this PR so that specific discussions about wording can take place.

Thanks.