Add the option to run a signer

[friedger]

As a signer, I want to configure in the .env file how my signer should run:

• private key
• prometheus api
• fees etc

[friedger]

This version of stacks-blockchain-docker has the api removed: https://github.com/friedger/stacks-local-dev/tree/signer

Just stacks-node + signer (+ prometeus)

[friedger]

The configuration for signer metrics introduced in #128 should be port 30001 not 9194 (compose-files/extra-service/signer.toml