Add `currencies/crypto` endpoint support

[astrohart]

@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 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