Add `currencies/crypto` endpoint support

@bchavez **Context**

What are you trying to do and how would you want to do it differently? Is it something you currently you cannot do? Is this related to an issue/problem?

Get a list of the crypto currencies supported by Coinbase by using its `/v2/currencies/crypto` endpoint, see [here](https://docs.cloud.coinbase.com/sign-in-with-coinbase/docs/api-currencies) and scroll down -- it talks about there being two endpoints.

**Alternatives**

I can always do a manual request using RestSharp and custom JSON objects, but it seems messy that way.

**Has the feature been requested before?**

No.

**If the feature request is approved, would you be willing to submit a PR?**

Yes