Create JSON file input schema

Create a [JSON schema](https://json-schema.org/) for the input JSON files. Related to #10

Example JSON (note that more fields could be added):
```json
{
"MSFT": {
"name": "Microsoft Corporation Common Stock",
"exchange": "NASDAQ-GS",
"sector": "Technology",
"industry": "Computer Software: Prepackaged Software",
"market_cap": 2280538394128
},
"AAPL": {
"name": "Apple Inc. Common Stock",
"exchange": "NASDAQ-GS",
"sector": "Technology",
"industry": "Computer Manufacturing",
"market_cap": 2942493344800
},
"GOOGL": {
"name": "Alphabet Inc. Class A Common Stock",
"exchange": "NASDAQ-GS",
"sector": "Technology",
"industry": "Internet and Information Services",
"market_cap": 1347658308378
}
}
```

All field names should conform to python `snake_case`.