Add ability to handle additional command-line arguments to the `contract_client!` macro

The contract_client! macro from the client_utils crate should have a way to handle additional applicatiton arguments on the command-line.

Details

The contract_client! macro creates a clap::App instance, parses all the arguments and creates the enclave API client all at once. Sometimes it would be useful to be able to give an additional argument to a client on the command-line (e.g. in clients/test-db-encryption/src/main.rs, the key manager's mrenclave could be provided on the command-line as is done elsewhere).

Acceptance Criteria

• The client_utils crate provides a way to specify additional command-line arguments when using the contract_client! macro.

Start in the client_utils crate and inspect the contract_client! macro, then compare its argument parsing with clients/test-db-encryption/src/main.rs and the other command-line usage mentioned in the issue. Define how callers can provide extra arguments while preserving the existing enclave API client setup, and verify the acceptance criterion with the relevant client or macro tests if present.