Add API Sandbox support

[NiklasRosenstein]

This PR adds support for the (Coinbase Advanced Trade API Sandbox](https://docs.cdp.coinbase.com/advanced-trade/docs/rest-api-sandbox) by

1. Adding a sandbox parameter to the RESTBase constructor.
2. Using that, if enabled, to let requests to unauthenticated to otherwise authenticated endpoints pass.
3. Falling back to the https://api-sandbox.coinbase.com base URL if sandbox is set to True and no explicit base URL is specified.

[cb-heimdall]

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