Add encoding/decoding of orchard extended spending keys

[AArnott]

On discord, dismad requested:

And will you provide access /show (perhaps as part of backup) https://zips.z.cash/zip-0032#orchard-extended-spending-keys

Searching the codebase for secret-orchard-extsk-main suggests we do not support orchard extended spending key encoding (for export or import of such keys). We should probably add it for completeness.