Support & Validate Bech32m-Encoded Payment Requests

`NUT-26` introduces a `Bech32m-based encodinh` for payment requests as an alternative to the existing CBOR+base64 format defined in `NUT-18` (see cashubtc/nuts#294).

While this proposal primarily affects wallet-side encoding and decoding, wel must remain compatible with Bech32m-encoded payment requests wherever the mint parses, validates, or persists payment request data.