CI/DevOps: Support separate release tags for signer binary

[kantai]

Now that this repo manages both the stacks-node and the signer, this repository is becoming something similar to a "monorepo". It makes sense that this repo should be able to support releases for the signer binary separate from the stacks-node, especially in 2.5.

The ultimate ask for this is a separate set of tags / releases for the signer -- basically, pushing a tag to this repo like signer-2.5.x and then uploading only the signer binary to that release page on github. Getting this to play nicely with CI will involve some work. In the interim, there may be some lower lift but more manual strategies for making this happen as needed.