Add NFC payload generation to `build.js` for the FR Medical Technician credential.

[BigBlueHat]

There's currently a data mismatch in https://github.com/credential-handler/vc-examples/blob/main/credentials/fr-medical-technician/credential.json#L88 because the values were added and changed by hand. We should make https://github.com/credential-handler/vc-examples/blob/89247f0ed2cc90cbee1623822acbf401aca4767f/credentials/fr-medical-technician/build.js use a template credential (similar to this one) which can be the "source of truth" for generating any static outputs (like the NFC payload).