Add support for third-party gas price estimator endpoints in the signer.

Based on CIP-18, we should provide a standardized interface for estimating gas usage and gas pricing for transactions. The signer currently relies on SimulateTx, which is resource-intensive. The proposed solution is to allow the signer to be configured with an external service that can provide such information by adding an additional option containing the URL of the third-party estimator(I guess https://celenium.io/ potentially can provide such info). If configured, the signer would query this information from the external service and if not, SimulateTx can be used as a fallback solution.