Add Rig Counts endpoints

## Overview
Add support for Baker Hughes rig count data.

## Endpoints to Add
- `GET /v1/rig-counts` - Get rig count data
- `GET /v1/rig-counts/latest` - Get latest rig count

## Types Needed
- `RigCountsResponse`
- `LatestRigCountResponse`

## Priority
**P1 - High Value** - Commonly requested data

## Swagger Reference
See `swagger/v1/swagger.json` in oilpriceapi-api repo for schema details