[Feature]: Support multiple Stacks node URLs

[djordon]

Feature - Support multiple Stacks node URLs

1. Description

1.1 Context & Purpose

Right now we only support one endpoint for getting nakamoto block data from a stacks node. Add support so that users can specify more than one endpoint for fetching stacks related data. Specifically, modify the code to expect a comma separated list of stacks node URLs in the signer's config.toml.

2. Technical Details:

2.1 Acceptance Criteria:

It's not clear if we'll want to always try the first endpoint in the list and fallback to the next one if that fails or do some kind of round robin load balancing. I'm leaning toward always trying the first one and falling back to the next one on connection/network errors since it is simpler.

3. Related Issues and Pull Requests (optional):

The original ticket for implementing the stacks client is #176.