[stacks-signer] Enable support for HTTPS with pinned certificate throughout the Stacks signer

[jferrant]

The stacks node itself does not use HTTPS however a user may want to set up a more secure environment and require a certificate to communicate to and from. Enable this throughout the stacks signer.

[jcnelson]

How is this going to help? The signer will need to be communicating with the node over a trusted network as it is (ideally a loopback interface), because any latency in the signer's ability to process events from the node will stall the node.