feat: add support for external signer

[joanestebanr]

Description

How to adopt new method of signing:

• Change the struct type types.KeystoreFileConfig for configuring the key to signer.SignerConfig
• Define the field Method to: local or web3signer. If not set is local
• The config file are 100% compatible with previous types.KeystoreFileConfig, so no user friction
• An old configuration like this is mapped to local:

{Path = "/app/keystore/claimsopnsor.keystore", Password = "testonly"}

• New web3signer: This use a server web3signer to store the keys an invoke the sign process using a API. To configure it you need to set:

{Method = "web3signer", URL = "http://localhost:8545", Address = "0x1234567890abcdef"}

• Method (mandatory): sign methor
• URL (mandatory): web3singer server
• Address (optional): Public key to use to sign, if this field is empty it try to guess the key to use, so if the server only have 1 key that this one is used.

Fixes #194

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
81.2% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud