Add support for AWS KMS as a way to sign block in the fuel-core

[xgreenx]

Overview

We need to add an ability to sign blocks with AWS KMS.

Implementation details

Use aws-sdk-kms crate for adding support for the PoA service to use AWS KMS to sign blocks.

It requires updating the CLI argument to pass a private key. It should be either a raw private key or credentials for KMS.

It would be nice to abstract the signing of the block via some trait and have different backed for signing. The sign function could be async.

Testing details

Please check the https://fuellabs.slack.com/archives/C0298A7TT1C/p1720046999839619 thread for test credentials to test the change with a real cluster.

[Dentosal]

Open questions:

• Should we allow passing in the configuration in any other way than just loading from the environment, i.e. with aws_config::load_from_env
  • Should we lock ourselves to a specific version of this config, or always use the latest availble defaults?
• Should we have this all behind an opt-out feature flag?