create GET public key method definition

[robdefeo]

Is your feature request related to a problem? Please describe.
The public key finder needs to expose a GET method to query the address and return a public key.

Describe the solution you'd like

• create pubkey.go in cmd/pkf/internal/http/handlers
• create skeleton methods based on cmd/mailchain/internal/http/handlers/pubkey.go
• include the swagger comments
• NOTE: implementation of the handler is not required for this issue.

Additional context
See cmd/mailchain/internal/http/handlers/pubkey.go for implementation in cmd/mailchain it is ok to copy from here to start with. Later there will be some drift as additional features are added.