GetAssetsRequest.AssetClass should be type AssetClass (instead of string)

There is already an [`AssetClass`](https://github.com/alpacahq/alpaca-trade-api-go/blob/7d20c8f30eaaf64619c4bd23b9855dd04dcccd35/alpaca/entities.go#L144) type, so it's just a matter of using it.

That is, change this:
```go
type GetAssetsRequest struct {
Status string
AssetClass string
Exchange string
}
```

To this:
```go
type GetAssetsRequest struct {
Status string
AssetClass AssetClass
Exchange string
}
```