Audit code and our deps for Prototype pollution

For reference https://learn.snyk.io/lessons/prototype-pollution/javascript/

> Prototype pollution is an injection attack that targets JavaScript runtimes. With prototype pollution, an attacker might control the default values of an object's properties. This allows the attacker to tamper with the logic of the application and can also lead to denial of service or, in extreme cases, remote code execution.

- [ ] Review Lodestar for unsafe deep merges and parsing. First target is the REST API
- [ ] Review dependent libraries, specially SSZ