Serve command for cmd/pkf

[robdefeo]

Is your feature request related to a problem? Please describe.
cmd/pkf needs a command to serve the API

Describe the solution you'd like
Create pkf serve command to serve the HTTP methods

Additional context
Depends on:

• create GET public key method definition #477
• implement public key finder using datastore  #479

[stale]

This issue has been automatically marked as stale because it has not had recent activity. The resources of the Mailchain team are limited, and so we are asking for your help. If this is a bug and you can still reproduce this error on the master branch, please reply in order to keep the issue open. If this is a feature request, and you feel that it is still relevant and valuable, please reply in order to keep the issue open. This issue will automatically be closed in the near future if no further activity occurs. Thank you for all your contributions.