Support for multi-tenancy

[hugueitor]

Multi-tenancy support is needed in EthSigner, when using Hyperledger Besu for each network deployed an EthSigner instance needs to be launched (with the corresponding use of resources).

In our company we provide infrastructure for blockchain networks for our customers, in our bussiness model for each customer we need to create his own private blockchain network, we are using Hashicorp Vault for key storage and management.

According to the EthSigner operational model for each network we have to start an EthSigner instance to manage the transactions for that particular network; in our case given that we have many customers they cannot share this EthSigner instance.

EthSigner being a Java app consumes a lot of resources and in our platfform is very restrictive the availabilitity of those resources, for that I would like to have a single EthSigner app that serves many networks by their ID's (for example).

At the moment the solution is to keep using Vault for storage and using a custom API that gets the cryptographic material and through Web3j sign and send transactions directly through the desired blockchain network.