[WIP] Adds Futures support (alpha)

[justinpolygon]

Added futures REST and WebSocket support (alpha). Includes new REST endpoints for futures data (e.g., aggregates, contracts, market statuses, etc) and WebSocket event parsing (e.g. trades, quotes, and aggregates).

REST examples:

package main

import (
	"context"
	"fmt"
	polygon "github.com/polygon-io/client-go/rest"
	"github.com/polygon-io/client-go/rest/models"
)

func main() {
	client := polygon.New("your_api_key")
	params := &models.ListFuturesAggsParams{
		Ticker:     "ESZ4",
		Multiplier: 1,
		Timespan:   "day",
		From:       models.Millis(time.Now().AddDate(0, 0, -7)),
		To:         models.Millis(time.Now()),
	}
	aggs := client.ListFuturesAggs(context.Background(), params)
	for aggs.Next() {
		fmt.Println(aggs.Item())
	}
	if aggs.Err() != nil {
		fmt.Println("Error:", aggs.Err())
	}
}

Websocket examples:

package main

import (
	"fmt"
	"log"
	polygonws "github.com/polygon-io/client-go/websocket"
)

func main() {
	config := polygonws.Config{
		APIKey: "your_api_key",
		Feed:   polygonws.RealTime,
		Market: polygonws.Futures,
	}
	client, err := polygonws.New(config)
	if err != nil {
		log.Fatal(err)
	}
	defer client.Close()

	err = client.Subscribe(polygonws.FuturesTrades, "ESZ4")
	if err != nil {
		log.Fatal(err)
	}

	err = client.Connect()
	if err != nil {
		log.Fatal(err)
	}

	for msg := range client.Output() {
		fmt.Println(msg)
	}
}

This is a work in progress PR and will get updates as things evolve.