Add GasPrice information

this is a follow-up and replaces #24

we should add a field "gasPrice" like this:
```json
{
"infoURL": "https://gasnow.org",
"eips": [ "1599" ],
"defaultValue": "2000000000"
}
```
Still thinking about a field that indicates if a gas price is stable "Under ordinary circumstances," like in xDAI
https://challenge.xdaichain.com/cost-estimates

Start by locating the chain metadata records and schema that define fields like infoURL, eips, and defaultValue, then compare them with the JSON shape in the issue. Review the linked xDAI discussion before deciding whether gasPrice should include a stability indicator. Done means the gasPrice field is represented consistently and the unresolved stability question has a documented decision.

Written by the indexing model from the issue text.