REST client examples

Really these examples serve as end-to-end tests for the REST client. These are not standard tests because they are should not be run as part of the standard cargo test infra, since they expect an Aptos API to be running already.

You can run examples like this, from the parent directory:

cargo run --example <dir>

For example:

cargo run --example account -- --api-url http://127.0.0.1:8080