Feature request: Allow setting shell command

For start-session it seems all commands are under ``sh`` on Linux, which is hard-coded and can't be overwritten. It would be good to be able to override this in both the SSM document for the session and the default in the ssm agent config.

Start by tracing the start-session handling and locating where the Linux shell command is hard-coded. Then inspect the SSM document inputs and the default in the SSM agent config. Done means both session documents and agent configuration can override the shell command, while the existing default behavior remains unchanged.

Written by the indexing model from the issue text.