Encode payjoin 2 subdirectory pubkey and ohttp= param in ur::bytewords

[DanGould]

base64Url encoding's character set require upper lowercasing characters, doubling the density of QRs which display their data. The only reason it was used was because it was readily available in the bitcoin crate and produced a String.

@ChristopherA suggested to use the UR encoding instead when a BIP for payjoin v2 was initially proposed. Neither the subdirectory public key identifier nore the ohttp param are themselves URIs, but they can still take advantage of the ur::bytewords encoding scheme in minimal style for QR compression benefits. Now that BIP 77 is finalizing we should seriously consider which encoding is best for the long haul.

UR's bytewords should result in smaller QR codes. I'll want to do some tests with actual QR codes but the idea is simple enough to implement thanks to @dspicher's library

[DanGould]

After some further research on encodings, I do believe minimal bytewords encodings is a good fit. Since both types of data are already structured and annotated by the bip21 uri that carries them, they don't need. It also provides a checksum which neither the default pubkey serialization nor Oblivious HTTP KeyConfiguration have themselves.