Add and export `validateConfig(input)` function

[Santiago Aceñolaza]

manager-multisig has a script to generate config json objects based on metadata stored in that repo. This generated json objects are meant to be used in this repo therefore it would be nice if this repo exports a function that validates these objects against the Zod schema defined here.

One caveat is that currently the Zod schema expects a json object with secrets already interpolated therefore we must refactor to allow validating config.json objects that have template strings instead of secrets.