Staring the sample pop server, keys shall be generate right away if not existent

[francis-pouatcha]

While starting the sample pop server it is bether server keys are initialized if not existent. currently:

• first call to the path http://localhost:3000/.well-known/did.json returns a default file
• then i http://localhost:3000/.well-known/did/pop.json?challenge=sfgasdfasdfasdfasf that run to a mistake because no keystore file
• then i run cargo run --bin didgen to generate a did
• then i http://localhost:3000/.well-known/did/pop.json?challenge=sfgasdfasdfasdfasf that return a verifiable credential of the server
• first call to the path http://localhost:3000/.well-known/did.json returns the corresponding file.