mock WKD should be returning unarmored keys

[tomholub]

I just noticed - we are returning armored keys here on the WKD endpoints but that does not reflect how WKD returns keys (which should be unarmored bytes, concatenated). I'll make another issue for this.

Originally posted by @tomholub in #3401 (comment)

OpenPGP.js has a method to return key bytes instead of the armored version. See example https://openpgpkey.flowcrypt.com/.well-known/openpgpkey/flowcrypt.com/hu/14bi5sf9whcm43n83ur8i6r4hfui5msk?l=tom

To return several keys, concatenate the bytes together.