[API] Entry point for certificate upload

[silenteh]

Once the AS id is agreed between the coordinator and the newly created account, then the scion local web interface needs to upload the certificate to the coordinator.

• Create an API entry point for the upload of the certificate.
• Simple validation of the certificate by checking that the AS id is present and correspond to the agreed one.

[ercanucan]

@shyamjvs @sivaprasadsudhir @ShantanuThakoor As per our discussion today, you can start looking at this issue as a first task once you have your environment setup.