Add local secret support for funder account so that signatory becomes optional

[jevonearth]

The tezos-key-gen-api relies on a signatory server to operate.

Request;

Add a new configuration property to the pools-config.json named funderSecret which should store a secret key.
The funderSecret should be mutually exclusive to the remoteSingerUrl. If both are present, a validation error should be thrown.

When funderSecret is present, tezos-key-gen-api can then sign operations in-process instead of relying in a signatory process. This makes it easier to operate the tezos-key-gen-api service in a sandbox based CI/CD or development workflow context where key security is not a concern.