Support `--agent` parameter for the `aish` executable

[StevenBucher98]

Summary of the new feature / enhancement

If I want to call aish non-interactively, it would be nice to be able to specify which agent instead of having to interactively choose which agent to talk to.

Proposed technical implementation details (optional)

aish --agent openai-gpt

aish -a openai-gpt

[RStadlmair]

I like the idea, but this should be in combination with a parameter in start-aishell.

I want: Start-AIShell -Agent openai-gpt