This change implements Web Key Directory lookup using user's e-mail
address. The target host is the same as the e-mail's domain and the
local-part is hashed with SHA-1 and encoded using Z-Base32 encoding.
Implemented is basic flow of version 06 of OpenPGP Web Key Directory
draft [0].
It was necessary to update node-fetch package to allow returning array
buffers from HTTP responses.
If openpgpjs is used in the browser all keys retrieved from Web Key
Directory should have `Access-Control-Allow-Origin` header set to `*`
(including 404 Not found responses).
[0]: https://datatracker.ietf.org/doc/draft-koch-openpgp-webkey-service/
