implement public key finder using datastore

[robdefeo]

Is your feature request related to a problem? Please describe.
cmd/pkf will return a public key based on an address, it should use the public_key.go in cmd/internal/datastore ref #478

Describe the solution you'd like

• implement the method as defined in create GET public key method definition #477
• use the datastore.PublicKey to search for the public key
• use datastore.PublicKey interface as parameter to handler creation
• return status 200 for not found public key, see cmd/mailchain/internal/http/handlers/resolve_name.go
• create unit tests