Add configuration for startRecordSessionCmd

When logging is enabled, ssm-agent uses `script` to record the session:

https://github.com/aws/amazon-ssm-agent/blob/57bb3c6935e12dce8cf662afffc435516d8dc3c3/agent/session/shell/shell_unix.go#L53

It would be great to be able to reconfigure this to use custom commands instead, to allow for more detailed recordings like [asciinema](https://asciinema.org/) or to add custom log entries.

Start in agent/session/shell/shell_unix.go at the linked startRecordSessionCmd usage and trace how session configuration is loaded. Determine where a custom recording command can be configured and how its arguments and output should be handled. Done means logging can use a configured command, including tools such as asciinema, while preserving session recording behavior.

Written by the indexing model from the issue text.