Implement support for Web Key Directory

[pravi]

Add support Web Key Directory Fixes #121

• generate wkd hash and store in database
• add options=wkd as supported query parameter
• search within wkdhash when options=wkd is passed
• add documentation for adding a rewrite rule to caddy web server to expose the url pattern expected by wkd clients.

Thanks to wkd-client npm module, Akshay S Dinesh and Ananthu CV