Add support for choice of salt for eip712 calls

[jot2re]

As it is now the salt for the eip712 domain used in signatures integrating with the KMS blockchain is set to None.
However, the most safe approach is probably to ensure that a custom, high entropy salt is used.
Still, it is not a high priority for now.

This needs to be done after issue https://github.com/zama-ai/kms-core/issues/1301